diff --git a/files_dedupe.go b/files_dedupe.go
index 73e0b83..bd3a3bf 100644
--- a/files_dedupe.go
+++ b/files_dedupe.go
@@ -154,6 +154,9 @@ func findWithinFolderDuplicates(indexed []ProjectFolderFile) []DedupGroup {
 			byKey[k] = append(byKey[k], it.File)
 		}
 		for k, group := range byKey {
+			if k == "" {
+				continue // dotfiles etc. have no stem: never treat as duplicates
+			}
 			if len(group) < 2 {
 				continue
 			}
@@ -178,6 +181,9 @@ func findCrossFolderDuplicates(indexed []ProjectFolderFile) []DedupGroup {
 	}
 	var out []DedupGroup
 	for k, items := range byKey {
+		if k == "" {
+			continue // dotfiles etc. have no stem: never treat as duplicates
+		}
 		if len(items) < 2 {
 			continue
 		}
diff --git a/files_dedupe_integration_test.go b/files_dedupe_integration_test.go
new file mode 100644
index 0000000..f71ce95
--- /dev/null
+++ b/files_dedupe_integration_test.go
@@ -0,0 +1,416 @@
+//go:build integration
+
+package onlyoffice
+
+import (
+	"context"
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"strconv"
+	"testing"
+	"time"
+)
+
+// TestIntegrationFileDedup proves on a live OnlyOffice portal that the file
+// dedup helpers find real duplicates and delete only the redundant copies.
+// It creates a throwaway "go-onlyoffice-test-" project (removed by cleanup),
+// places same stem|ext files in two subfolders and in the project root, then
+// exercises FindProjectDuplicates, mergeProjectRootForDedupe,
+// ApplyDedupGroups and DeleteFilesByDedupKey. Destructive — run only against
+// an instance you own.
+func TestIntegrationFileDedup(t *testing.T) {
+	c := liveClient(t)
+	t.Cleanup(func() { cleanupTestProjects(t, c) })
+	ctx := context.Background()
+
+	suffix := time.Now().UTC().Format("20060102-150405")
+	project, err := c.CreateProject(NewProjectRequest{
+		Title:       testProjectPrefix + "dedup-" + suffix,
+		Description: "go-onlyoffice file dedup integration",
+	})
+	if err != nil {
+		t.Fatalf("CreateProject: %v", err)
+	}
+	if project.ID == nil {
+		t.Fatal("created project without id")
+	}
+	pid := strconv.Itoa(*project.ID)
+
+	root := projectFolderEventually(t, ctx, c, pid)
+
+	aID := createFolderLive(t, ctx, c, root, "A-"+suffix)
+	bID := createFolderLive(t, ctx, c, root, "B-"+suffix)
+	createFolderLive(t, ctx, c, root, "_trash-"+suffix)
+
+	// Check IsTrashFolderTitle against a real live folder title.
+	if !IsTrashFolderTitle("_trash-" + suffix) {
+		t.Fatalf("IsTrashFolderTitle(%q) = false for a live _trash folder", "_trash-"+suffix)
+	}
+
+	stem := "dedup-" + suffix
+	local := writeLocalFile(t, stem+".txt", []byte("dedup integration "+suffix+"\n"))
+
+	a1 := uploadFolderLive(t, ctx, c, aID, local)
+	a2 := uploadFolderLive(t, ctx, c, aID, local)
+	b1 := uploadFolderLive(t, ctx, c, bID, local)
+	r1 := uploadFolderLive(t, ctx, c, root, local)
+	r2 := uploadFolderLive(t, ctx, c, root, local)
+	t.Logf("uploaded a1=%d a2=%d b1=%d r1=%d r2=%d",
+		FileEntryNumericID(a1), FileEntryNumericID(a2), FileEntryNumericID(b1),
+		FileEntryNumericID(r1), FileEntryNumericID(r2))
+
+	if n := dedupWaitCount(t, ctx, c, aID, stem, ".txt", 2, 30*time.Second); n != 2 {
+		t.Fatalf("folder A has %d copies after upload, want 2", n)
+	}
+	if n := dedupWaitCount(t, ctx, c, bID, stem, ".txt", 1, 30*time.Second); n != 1 {
+		t.Fatalf("folder B has %d copies after upload, want 1", n)
+	}
+	if n := dedupWaitCount(t, ctx, c, root, stem, ".txt", 2, 30*time.Second); n != 2 {
+		t.Fatalf("project root has %d copies after upload, want 2", n)
+	}
+
+	// #2 mergeProjectRootForDedupe on the live tree: the project root that
+	// carries documents must be part of the scan exactly once.
+	folders, byFolder, rootFiles := liveProjectIndex(t, ctx, c, pid, root)
+	merged, mergedBy := mergeProjectRootForDedupe(root, folders, byFolder, rootFiles)
+	rootCount := 0
+	for _, folder := range merged {
+		if folder != nil && folder.ID != nil && folder.ID.String() == root {
+			rootCount++
+		}
+	}
+	if rootCount != 1 {
+		t.Fatalf("mergeProjectRootForDedupe: project root appears %d times in live tree, want 1", rootCount)
+	}
+	if got := len(FindFilesByDedupKey(mergedBy[root], stem, ".txt")); got != 2 {
+		t.Fatalf("mergeProjectRootForDedupe: root carries %d matching files, want 2", got)
+	}
+
+	// Within-folder scan: a duplicate pair in A and in the project root.
+	within := FindProjectDuplicates(merged, mergedBy, DedupOptions{})
+	removesByFolder := map[string]int{}
+	for _, g := range within {
+		removesByFolder[g.FolderID] = len(g.Remove)
+	}
+	if len(within) != 2 || removesByFolder[aID] != 1 || removesByFolder[root] != 1 {
+		t.Fatalf("within-folder groups = %d (%v), want exactly A:1 root:1", len(within), removesByFolder)
+	}
+
+	// DeleteFilesByDedupKey removes every stem|ext copy in one folder.
+	removed, err := c.DeleteFilesByDedupKey(ctx, aID, stem, ".txt")
+	if err != nil {
+		t.Fatalf("DeleteFilesByDedupKey(A): %v", err)
+	}
+	if len(removed) != 2 {
+		t.Fatalf("DeleteFilesByDedupKey(A) removed %v, want 2 ids", removed)
+	}
+	if n := dedupWaitCount(t, ctx, c, aID, stem, ".txt", 0, 30*time.Second); n != 0 {
+		t.Fatalf("folder A still has %d copies after DeleteFilesByDedupKey", n)
+	}
+
+	// Re-create the A duplicates so the cross-folder project scan can be
+	// applied and a single survivor proven by polling.
+	uploadFolderLive(t, ctx, c, aID, local)
+	uploadFolderLive(t, ctx, c, aID, local)
+	if n := dedupWaitCount(t, ctx, c, aID, stem, ".txt", 2, 30*time.Second); n != 2 {
+		t.Fatalf("folder A has %d re-uploaded copies, want 2", n)
+	}
+
+	// Cross-folder dry-run over the live project: one key, five copies, and
+	// the project-root copies must be part of the group (root merge live).
+	groups, deleted, err := dedupeProjectEventually(t, ctx, c, pid, DedupOptions{CrossFolder: true}, false)
+	if err != nil {
+		t.Fatalf("DedupeProject: %v", err)
+	}
+	if len(deleted) != 0 {
+		t.Fatalf("dry-run DedupeProject deleted %v", deleted)
+	}
+	if len(groups) != 1 {
+		t.Fatalf("DedupeProject cross-folder groups = %d, want 1 (%+v)", len(groups), groups)
+	}
+	if len(groups[0].Remove) != 4 {
+		t.Fatalf("cross-folder group removes %d files, want 4", len(groups[0].Remove))
+	}
+	if !dedupGroupHasID(groups[0], FileEntryNumericID(r1)) && !dedupGroupHasID(groups[0], FileEntryNumericID(r2)) {
+		t.Fatalf("cross-folder group does not include a project-root copy (merge not applied)")
+	}
+
+	keepID := FileEntryNumericID(groups[0].Keep)
+	deleted, err = c.ApplyDedupGroups(ctx, groups)
+	if err != nil {
+		t.Fatalf("ApplyDedupGroups: %v", err)
+	}
+	if len(deleted) != 4 {
+		t.Fatalf("ApplyDedupGroups deleted %v, want 4 ids", deleted)
+	}
+
+	survivorID, total := dedupWaitTotal(t, ctx, c, []string{aID, bID, root}, stem, ".txt", 1, 40*time.Second)
+	if total != 1 {
+		t.Fatalf("after ApplyDedupGroups %d copies survive, want 1", total)
+	}
+	if survivorID != keepID {
+		t.Fatalf("remaining copy id = %d, want kept id %d", survivorID, keepID)
+	}
+
+	// The removed ids must really be gone from every folder.
+	gone := map[int64]bool{
+		FileEntryNumericID(a2): true,
+		FileEntryNumericID(b1): true,
+		FileEntryNumericID(r1): true,
+		FileEntryNumericID(r2): true,
+	}
+	for _, fid := range []string{aID, bID, root} {
+		files, err := c.FolderFiles(ctx, fid)
+		if err != nil {
+			t.Fatalf("FolderFiles %s: %v", fid, err)
+		}
+		for _, f := range FindFilesByDedupKey(files, stem, ".txt") {
+			id := FileEntryNumericID(f)
+			if gone[id] {
+				t.Fatalf("removed id %d still present in folder %s", id, fid)
+			}
+		}
+	}
+	t.Logf("survivor id=%d keep id=%d, deleted=%v", survivorID, keepID, deleted)
+}
+
+// createFolderLive creates a subfolder (retrying transient 5xx) and returns
+// its Documents folder id.
+func createFolderLive(t *testing.T, ctx context.Context, c *Client, parentID, title string) string {
+	t.Helper()
+	deadline := time.Now().Add(30 * time.Second)
+	var (
+		m   map[string]any
+		err error
+	)
+	for {
+		m, err = c.CreateFolder(ctx, parentID, title)
+		if err == nil || time.Now().After(deadline) {
+			break
+		}
+		time.Sleep(time.Second)
+	}
+	if err != nil {
+		t.Fatalf("CreateFolder %q: %v", title, err)
+	}
+	if m == nil {
+		t.Fatalf("CreateFolder %q: empty response", title)
+	}
+	switch v := m["id"].(type) {
+	case float64:
+		return strconv.FormatInt(int64(v), 10)
+	case json.Number:
+		return v.String()
+	case string:
+		if v != "" {
+			return v
+		}
+	}
+	t.Fatalf("CreateFolder %q: no id in response %#v", title, m)
+	return ""
+}
+
+// writeLocalFile writes content to a temp file and returns its path.
+func writeLocalFile(t *testing.T, name string, content []byte) string {
+	t.Helper()
+	p := filepath.Join(t.TempDir(), name)
+	if err := os.WriteFile(p, content, 0o600); err != nil {
+		t.Fatal(err)
+	}
+	return p
+}
+
+// uploadFolderLive uploads localPath into folderID (retrying the portal's
+// transient post-create 500) and returns the file entry.
+func uploadFolderLive(t *testing.T, ctx context.Context, c *Client, folderID, localPath string) *FileEntry {
+	t.Helper()
+	deadline := time.Now().Add(30 * time.Second)
+	var (
+		e   *FileEntry
+		err error
+	)
+	for {
+		e, err = c.UploadToFolder(ctx, folderID, localPath)
+		if err == nil || time.Now().After(deadline) {
+			break
+		}
+		time.Sleep(time.Second)
+	}
+	if err != nil {
+		t.Fatalf("UploadToFolder %s: %v", folderID, err)
+	}
+	if e == nil || e.ID == nil {
+		t.Fatalf("UploadToFolder %s: no file entry (%+v)", folderID, e)
+	}
+	return e
+}
+
+// liveProjectIndex rebuilds the project folder/file index the same way
+// DedupeProject does, for direct mergeProjectRootForDedupe assertions.
+func liveProjectIndex(t *testing.T, ctx context.Context, c *Client, projectID, rootID string) ([]*FolderEntry, map[string][]*FileEntry, []*FileEntry) {
+	t.Helper()
+	pf := getProjectFilesEventually(t, ctx, c, projectID)
+	rootFiles := folderFilesEventually(t, ctx, c, rootID)
+	folders := make([]*FolderEntry, 0, len(pf.Folders)+1)
+	byFolder := make(map[string][]*FileEntry, len(pf.Folders)+1)
+	for _, folder := range pf.Folders {
+		if folder == nil || folder.ID == nil {
+			continue
+		}
+		fid := folder.ID.String()
+		if fid == rootID {
+			byFolder[fid] = rootFiles
+		} else {
+			byFolder[fid] = folderFilesEventually(t, ctx, c, fid)
+		}
+		folders = append(folders, folder)
+	}
+	return folders, byFolder, rootFiles
+}
+
+// projectFolderEventually resolves the project Documents root id, retrying on
+// a transient portal answer.
+func projectFolderEventually(t *testing.T, ctx context.Context, c *Client, projectID string) string {
+	t.Helper()
+	deadline := time.Now().Add(30 * time.Second)
+	var (
+		root string
+		err  error
+	)
+	for {
+		root, err = c.projectFolderID(ctx, projectID)
+		if err == nil || time.Now().After(deadline) {
+			break
+		}
+		time.Sleep(time.Second)
+	}
+	if err != nil {
+		t.Fatalf("projectFolderID: %v", err)
+	}
+	if root == "" {
+		t.Fatal("projectFolderID returned empty id")
+	}
+	return root
+}
+
+// getProjectFilesEventually lists a project's files/folders, retrying on a
+// transient portal answer.
+func getProjectFilesEventually(t *testing.T, ctx context.Context, c *Client, projectID string) *ProjectFilesResponse {
+	t.Helper()
+	deadline := time.Now().Add(30 * time.Second)
+	var last error
+	for {
+		pf, err := c.GetProjectFiles(ctx, projectID)
+		if err == nil {
+			return pf
+		}
+		last = err
+		if time.Now().After(deadline) {
+			t.Fatalf("GetProjectFiles %s: %v", projectID, last)
+		}
+		time.Sleep(500 * time.Millisecond)
+	}
+}
+
+// folderFilesEventually lists a folder, retrying while the portal answers
+// transiently (a freshly created folder can 500 until its parent map settles).
+func folderFilesEventually(t *testing.T, ctx context.Context, c *Client, folderID string) []*FileEntry {
+	t.Helper()
+	deadline := time.Now().Add(30 * time.Second)
+	var last error
+	for {
+		files, err := c.FolderFiles(ctx, folderID)
+		if err == nil {
+			return files
+		}
+		last = err
+		if time.Now().After(deadline) {
+			t.Fatalf("FolderFiles %s: %v", folderID, last)
+		}
+		time.Sleep(500 * time.Millisecond)
+	}
+}
+
+// dedupeProjectEventually runs a project dedup scan, retrying the whole scan on
+// a transient portal error. It is used for dry-runs only (apply must stay a
+// single deliberate call).
+func dedupeProjectEventually(t *testing.T, ctx context.Context, c *Client, projectID string, opts DedupOptions, apply bool) ([]DedupGroup, []int, error) {
+	t.Helper()
+	deadline := time.Now().Add(30 * time.Second)
+	var (
+		groups  []DedupGroup
+		deleted []int
+		err     error
+	)
+	for {
+		groups, deleted, err = c.DedupeProject(ctx, projectID, opts, apply)
+		if err == nil || time.Now().After(deadline) {
+			return groups, deleted, err
+		}
+		time.Sleep(time.Second)
+	}
+}
+
+// dedupWaitCount polls folderID until want stem|ext copies are visible.
+func dedupWaitCount(t *testing.T, ctx context.Context, c *Client, folderID, stem, ext string, want int, d time.Duration) int {
+	t.Helper()
+	deadline := time.Now().Add(d)
+	got := -1
+	for {
+		if files, err := c.FolderFiles(ctx, folderID); err == nil {
+			got = len(FindFilesByDedupKey(files, stem, ext))
+			if got == want {
+				return got
+			}
+		}
+		if time.Now().After(deadline) {
+			return got
+		}
+		time.Sleep(500 * time.Millisecond)
+	}
+}
+
+// dedupWaitTotal polls the given folders until the total number of stem|ext
+// copies reaches want, returning the last seen file id and count.
+func dedupWaitTotal(t *testing.T, ctx context.Context, c *Client, folderIDs []string, stem, ext string, want int, d time.Duration) (int64, int) {
+	t.Helper()
+	deadline := time.Now().Add(d)
+	var survivor int64
+	total := -1
+	for {
+		survivor, total = 0, 0
+		for _, fid := range folderIDs {
+			files, err := c.FolderFiles(ctx, fid)
+			if err != nil {
+				continue
+			}
+			for _, f := range FindFilesByDedupKey(files, stem, ext) {
+				total++
+				survivor = FileEntryNumericID(f)
+			}
+		}
+		if total == want {
+			return survivor, total
+		}
+		if time.Now().After(deadline) {
+			return survivor, total
+		}
+		time.Sleep(500 * time.Millisecond)
+	}
+}
+
+func dedupGroupHasID(g DedupGroup, id int64) bool {
+	if id == 0 {
+		return false
+	}
+	if FileEntryNumericID(g.Keep) == id {
+		return true
+	}
+	for _, f := range g.Remove {
+		if FileEntryNumericID(f) == id {
+			return true
+		}
+	}
+	return false
+}
diff --git a/files_dedupe_test.go b/files_dedupe_test.go
index 6d24a85..f75d2a3 100644
--- a/files_dedupe_test.go
+++ b/files_dedupe_test.go
@@ -84,6 +84,32 @@ func TestMergeProjectRootForDedupe(t *testing.T) {
 	}
 }
 
+func TestFindDuplicatesSkipsEmptyKey(t *testing.T) {
+	// Dotfiles (".env", ".gitignore", ".npmrc") normalize to an empty stem, so
+	// FileDedupKey is "". They are not duplicates of each other and must never
+	// form a dedup group that would delete one of them.
+	env := &FileEntry{ID: jsonNum("1"), Title: strPtr(".env")}
+	gitignore := &FileEntry{ID: jsonNum("2"), Title: strPtr(".gitignore")}
+	npmrc := &FileEntry{ID: jsonNum("3"), Title: strPtr(".npmrc")}
+	if FileDedupKey(env) != "" || FileDedupKey(gitignore) != "" || FileDedupKey(npmrc) != "" {
+		t.Fatalf("dotfiles should have empty dedup key")
+	}
+	within := []ProjectFolderFile{
+		{FolderID: "500", FolderTitle: "Cfg", File: env},
+		{FolderID: "500", FolderTitle: "Cfg", File: gitignore},
+	}
+	if groups := findWithinFolderDuplicates(within); len(groups) != 0 {
+		t.Fatalf("within-folder empty-key groups = %d, want 0 (%+v)", len(groups), groups)
+	}
+	cross := []ProjectFolderFile{
+		{FolderID: "500", FolderTitle: "Cfg", File: env},
+		{FolderID: "501", FolderTitle: "Other", File: npmrc},
+	}
+	if groups := findCrossFolderDuplicates(cross); len(groups) != 0 {
+		t.Fatalf("cross-folder empty-key groups = %d, want 0 (%+v)", len(groups), groups)
+	}
+}
+
 func TestIsTrashFolderTitle(t *testing.T) {
 	if !IsTrashFolderTitle("_trash-md") {
 		t.Fatal("expected trash")
diff --git a/files_webdav.go b/files_webdav.go
index 197c394..a38b732 100644
--- a/files_webdav.go
+++ b/files_webdav.go
@@ -243,18 +243,27 @@ func fileopsError(raw json.RawMessage) error {
 func (c *Client) DeleteDavItems(ctx context.Context, folderIDs, fileIDs []string) error {
 	body := map[string]any{"DeleteAfter": true, "Immediately": true}
 	for _, id := range folderIDs {
-		if _, err := c.deleteJSON(ctx, "/api/2.0/files/folder/"+url.PathEscape(id), body); err != nil {
+		if err := c.deleteDavItem(ctx, "/api/2.0/files/folder/"+url.PathEscape(id), body); err != nil {
 			return err
 		}
 	}
 	for _, id := range fileIDs {
-		if _, err := c.deleteJSON(ctx, "/api/2.0/files/file/"+url.PathEscape(id), body); err != nil {
+		if err := c.deleteDavItem(ctx, "/api/2.0/files/file/"+url.PathEscape(id), body); err != nil {
 			return err
 		}
 	}
 	return nil
 }
 
+// deleteDavItem deletes one item, retrying transient 429/502/503/504 answers
+// through DoRetry like every other bulk path (deletes are idempotent).
+func (c *Client) deleteDavItem(ctx context.Context, path string, body any) error {
+	return DoRetry(ctx, DefaultRetryPolicy(), func() error {
+		_, err := c.deleteJSON(ctx, path, body)
+		return err
+	})
+}
+
 // UploadDavFile uploads src (fileName) into folderID, streaming from src.
 //
 // Deprecated: use FileStore.Upload via Client.Files()/Client.FileStore.