diff --git a/AGENTS.md b/AGENTS.md
index a9b9e31..489fd7b 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -28,7 +28,7 @@ Canonical Go client for OnlyOffice Workspace (Projects + Calendar + CRM) and the
 - Prefer `ResponseObject` / `postFormObject` / `putFormObject` / `deleteObject` over hand-rolled `json.Unmarshal(responseField(...))` blocks — they exist for DRY, use them.
 - Domain split is by file, **not** by subpackage. Don't introduce `internal/` or `pkg/*` subpackages inside the library — it flattens the `*Client` call surface for a reason.
 - CLI commands follow **subject → verb** structure (`oo <subject> <verb>`), never `oo <verb>-<subject>`. Add new commands to the existing subject file if one fits; create a new `cmd/oo/<subject>.go` for a genuinely new domain.
-- **Documents for agents:** prefer Markdown in git; OnlyOffice UI is weak for `.md`/`.txt`. Use `oo docs put-md` (md→docx) and `oo docs put-txt` (txt→docx, preserves line breaks). `oo projects files dedupe PROJECT_ID` reports/removes duplicate stem|ext copies (`--apply`, `--cross`).
+- **Documents for agents:** prefer Markdown in git; OnlyOffice UI is weak for `.md`/`.txt`. Use `oo docs put-md` (md→docx) and `oo docs put-txt` (txt→docx, preserves line breaks). All upload paths default to **upsert** by `stem|ext` (`--replace`, default true); `--no-replace` fails on conflict; `--allow-duplicate` opts into raw OO append. `oo projects files dedupe PROJECT_ID` reports/removes duplicate stem|ext copies (`--apply`, `--cross`; includes project root folder).
 - Every table output goes through `printTable(headers, rows)`; every single-object through `printObject(v)`. Do not `fmt.Println` rows ad-hoc or the `--output json` flag breaks for that command.
 - No secrets in the repo; use `.env` (gitignored). Commit `.env.example` only.
 - Follow SemVer on tags; this repo is tagged at GitHub under `git@github.com:eSlider/go-onlyoffice.git`.
diff --git a/cmd/oo/docs.go b/cmd/oo/docs.go
index 35b4b6c..7e7ff31 100644
--- a/cmd/oo/docs.go
+++ b/cmd/oo/docs.go
@@ -1,6 +1,7 @@
 package main
 
 import (
+	"context"
 	"fmt"
 	"os"
 	"path/filepath"
@@ -423,44 +424,28 @@ func docsPutMDCmd() *cobra.Command {
 				}
 			}
 			ctx := cmd.Context()
-			var ent *onlyoffice.FileEntry
-			var deleted []int
-			if folderID != "" {
-				if replace {
-					ent, deleted, err = c.UploadToFolderReplacing(ctx, folderID, docxPath)
-				} else {
-					ent, err = c.UploadToFolder(ctx, folderID, docxPath)
-				}
-				if err != nil {
-					return err
-				}
-				obj := map[string]any{
-					"project_id": pid,
-					"folder_id":  folderID,
-					"md":         mdPath,
-					"uploaded":   fileEntryToMap(ent),
-				}
-				if len(deleted) > 0 {
-					obj["replaced_file_ids"] = deleted
-				}
-				printObject(obj)
-				return nil
-			}
-			ent, err = c.UploadProjectFile(ctx, pid, docxPath)
+			ent, deleted, err := uploadProjectDoc(ctx, c, pid, docxPath, folderID, replace)
 			if err != nil {
 				return err
 			}
-			printObject(map[string]any{
+			obj := map[string]any{
 				"project_id": pid,
 				"md":         mdPath,
 				"uploaded":   fileEntryToMap(ent),
-			})
+			}
+			if folderID != "" {
+				obj["folder_id"] = folderID
+			}
+			if len(deleted) > 0 {
+				obj["replaced_file_ids"] = deleted
+			}
+			printObject(obj)
 			return nil
 		},
 	}
 	cmd.Flags().StringVar(&folderID, "folder", "", "Documents folder id (default: project root)")
 	cmd.Flags().StringVar(&keepLocalDOCX, "keep-docx", "", "also write the generated DOCX to this local path")
-	cmd.Flags().BoolVar(&replace, "replace", true, "delete same-stem files in folder before upload (put-md upsert)")
+	cmd.Flags().BoolVar(&replace, "replace", true, "replace same stem|ext before upload (default); false = fail if name taken")
 	return cmd
 }
 
@@ -503,44 +488,28 @@ func docsPutTxtCmd() *cobra.Command {
 				}
 			}
 			ctx := cmd.Context()
-			var ent *onlyoffice.FileEntry
-			var deleted []int
-			if folderID != "" {
-				if replace {
-					ent, deleted, err = c.UploadToFolderReplacing(ctx, folderID, docxPath)
-				} else {
-					ent, err = c.UploadToFolder(ctx, folderID, docxPath)
-				}
-				if err != nil {
-					return err
-				}
-				obj := map[string]any{
-					"project_id": pid,
-					"txt":        txtPath,
-					"folder_id":  folderID,
-					"uploaded":   fileEntryToMap(ent),
-				}
-				if len(deleted) > 0 {
-					obj["replaced_file_ids"] = deleted
-				}
-				printObject(obj)
-				return nil
-			}
-			ent, err = c.UploadProjectFile(ctx, pid, docxPath)
+			ent, deleted, err := uploadProjectDoc(ctx, c, pid, docxPath, folderID, replace)
 			if err != nil {
 				return err
 			}
-			printObject(map[string]any{
+			obj := map[string]any{
 				"project_id": pid,
 				"txt":        txtPath,
 				"uploaded":   fileEntryToMap(ent),
-			})
+			}
+			if folderID != "" {
+				obj["folder_id"] = folderID
+			}
+			if len(deleted) > 0 {
+				obj["replaced_file_ids"] = deleted
+			}
+			printObject(obj)
 			return nil
 		},
 	}
 	cmd.Flags().StringVar(&folderID, "folder", "", "Documents folder id (default: project root)")
 	cmd.Flags().StringVar(&keepLocalDOCX, "keep-docx", "", "also write the generated DOCX to this local path")
-	cmd.Flags().BoolVar(&replace, "replace", true, "delete same-stem files in folder before upload (put-txt upsert)")
+	cmd.Flags().BoolVar(&replace, "replace", true, "replace same stem|ext before upload (default); false = fail if name taken")
 	return cmd
 }
 
@@ -618,21 +587,7 @@ formulas (SUM/AVG, cross-sheet refs, named inputs) via --template, or upload an
 			}
 
 			ctx := cmd.Context()
-			var ent *onlyoffice.FileEntry
-			var deleted []int
-			if folderID != "" {
-				if replace {
-					ent, deleted, err = c.UploadToFolderReplacing(ctx, folderID, xlsxPath)
-				} else {
-					ent, err = c.UploadToFolder(ctx, folderID, xlsxPath)
-				}
-			} else {
-				if replace {
-					ent, deleted, err = c.UploadProjectFileReplacing(ctx, pid, xlsxPath)
-				} else {
-					ent, err = c.UploadProjectFile(ctx, pid, xlsxPath)
-				}
-			}
+			ent, deleted, err := uploadProjectDoc(ctx, c, pid, xlsxPath, folderID, replace)
 			if err != nil {
 				return err
 			}
@@ -655,6 +610,25 @@ formulas (SUM/AVG, cross-sheet refs, named inputs) via --template, or upload an
 	cmd.Flags().StringVar(&template, "template", "", "built-in workbook template (cutover-portugal)")
 	cmd.Flags().StringVar(&title, "title", "", "upload file name when using --template")
 	cmd.Flags().StringVar(&keepLocal, "keep-xlsx", "", "also write generated/uploaded bytes to this local path")
-	cmd.Flags().BoolVar(&replace, "replace", true, "delete same-stem files before upload")
+	cmd.Flags().BoolVar(&replace, "replace", true, "replace same stem|ext before upload (default); false = fail if name taken")
 	return cmd
 }
+
+// uploadProjectDoc upserts (--replace, default) or no-clobbers into project/folder Documents.
+func uploadProjectDoc(ctx context.Context, c *onlyoffice.Client, pid, localPath, folderID string, replace bool) (*onlyoffice.FileEntry, []int, error) {
+	if folderID != "" {
+		if replace {
+			return c.UploadToFolderReplacing(ctx, folderID, localPath)
+		}
+		if err := c.AssertNoFileConflict(ctx, folderID, localPath); err != nil {
+			return nil, nil, err
+		}
+		ent, err := c.UploadToFolder(ctx, folderID, localPath)
+		return ent, nil, err
+	}
+	if replace {
+		return c.UploadProjectFileReplacing(ctx, pid, localPath)
+	}
+	ent, err := c.UploadProjectFileNoClobber(ctx, pid, localPath)
+	return ent, nil, err
+}
diff --git a/cmd/oo/projects_files.go b/cmd/oo/projects_files.go
index df726d1..0469d98 100644
--- a/cmd/oo/projects_files.go
+++ b/cmd/oo/projects_files.go
@@ -107,10 +107,13 @@ func prjFilesListCmd() *cobra.Command {
 }
 
 func prjFilesUploadCmd() *cobra.Command {
-	return &cobra.Command{
+	var replace, allowDuplicate bool
+	cmd := &cobra.Command{
 		Use:   "upload PROJECT_ID LOCAL_PATH [LOCAL_PATH...]",
-		Short: "Upload file(s) into the project's Documents folder",
-		Args:  cobra.MinimumNArgs(2),
+		Short: "Upload file(s) into the project's Documents folder (upsert by stem|ext)",
+		Long: `Default: replace an existing file with the same logical name (stem|ext), like cp overwrite.
+Pass --no-replace to fail when the name is taken; --allow-duplicate to always create a new file id.`,
+		Args: cobra.MinimumNArgs(2),
 		RunE: func(cmd *cobra.Command, args []string) error {
 			c, err := newOO(cmd)
 			if err != nil {
@@ -118,15 +121,31 @@ func prjFilesUploadCmd() *cobra.Command {
 			}
 			pid := args[0]
 			for _, p := range args[1:] {
-				entry, err := c.UploadProjectFile(cmd.Context(), pid, p)
+				var entry *onlyoffice.FileEntry
+				var deleted []int
+				switch {
+				case allowDuplicate:
+					entry, err = c.UploadProjectFile(cmd.Context(), pid, p)
+				case replace:
+					entry, deleted, err = c.UploadProjectFileReplacing(cmd.Context(), pid, p)
+				default:
+					entry, err = c.UploadProjectFileNoClobber(cmd.Context(), pid, p)
+				}
 				if err != nil {
 					return err
 				}
-				printObject(fileEntryToMap(entry))
+				obj := fileEntryToMap(entry)
+				if len(deleted) > 0 {
+					obj["replaced_file_ids"] = deleted
+				}
+				printObject(obj)
 			}
 			return nil
 		},
 	}
+	cmd.Flags().BoolVar(&replace, "replace", true, "replace same stem|ext in project folder (default)")
+	cmd.Flags().BoolVar(&allowDuplicate, "allow-duplicate", false, "always create a new file even when the name exists")
+	return cmd
 }
 
 func prjFilesDownloadCmd() *cobra.Command {
diff --git a/files_dedupe.go b/files_dedupe.go
index c0161a2..73e0b83 100644
--- a/files_dedupe.go
+++ b/files_dedupe.go
@@ -2,6 +2,7 @@ package onlyoffice
 
 import (
 	"context"
+	"encoding/json"
 	"path/filepath"
 	"sort"
 	"strings"
@@ -310,25 +311,70 @@ func (c *Client) DeleteFilesByDedupKey(ctx context.Context, folderID, stem, ext
 	return ids, nil
 }
 
+// mergeProjectRootForDedupe includes projectFolder files in dedupe scans. OO often lists
+// root documents only in pf.Files while pf.Folders is empty.
+func mergeProjectRootForDedupe(rootID string, folders []*FolderEntry, filesByFolder map[string][]*FileEntry, rootFiles []*FileEntry) ([]*FolderEntry, map[string][]*FileEntry) {
+	if rootID == "" {
+		return folders, filesByFolder
+	}
+	if filesByFolder == nil {
+		filesByFolder = map[string][]*FileEntry{}
+	}
+	for _, folder := range folders {
+		if folder != nil && folder.ID != nil && folder.ID.String() == rootID {
+			if len(rootFiles) > 0 {
+				filesByFolder[rootID] = rootFiles
+			}
+			return folders, filesByFolder
+		}
+	}
+	if len(rootFiles) == 0 {
+		return folders, filesByFolder
+	}
+	id := json.Number(rootID)
+	title := "(project root)"
+	folders = append(folders, &FolderEntry{ID: &id, Title: &title})
+	filesByFolder[rootID] = rootFiles
+	return folders, filesByFolder
+}
+
 // DedupeProject scans project folders and optionally deletes duplicates.
 func (c *Client) DedupeProject(ctx context.Context, projectID string, opts DedupOptions, apply bool) ([]DedupGroup, []int, error) {
 	pf, err := c.GetProjectFiles(ctx, projectID)
 	if err != nil {
 		return nil, nil, err
 	}
-	filesByFolder := make(map[string][]*FileEntry, len(pf.Folders))
+	rootID, err := c.projectFolderID(ctx, projectID)
+	if err != nil {
+		return nil, nil, err
+	}
+	var rootFiles []*FileEntry
+	if rootID != "" {
+		rootFiles, err = c.FolderFiles(ctx, rootID)
+		if err != nil {
+			return nil, nil, err
+		}
+	}
+	filesByFolder := make(map[string][]*FileEntry, len(pf.Folders)+1)
+	folders := make([]*FolderEntry, 0, len(pf.Folders)+1)
 	for _, folder := range pf.Folders {
 		if folder == nil || folder.ID == nil {
 			continue
 		}
 		fid := folder.ID.String()
-		files, err := c.FolderFiles(ctx, fid)
-		if err != nil {
-			return nil, nil, err
+		if fid == rootID {
+			filesByFolder[fid] = rootFiles
+		} else {
+			files, err := c.FolderFiles(ctx, fid)
+			if err != nil {
+				return nil, nil, err
+			}
+			filesByFolder[fid] = files
 		}
-		filesByFolder[fid] = files
+		folders = append(folders, folder)
 	}
-	groups := FindProjectDuplicates(pf.Folders, filesByFolder, opts)
+	folders, filesByFolder = mergeProjectRootForDedupe(rootID, folders, filesByFolder, rootFiles)
+	groups := FindProjectDuplicates(folders, filesByFolder, opts)
 	if !apply || len(groups) == 0 {
 		return groups, nil, nil
 	}
diff --git a/files_dedupe_test.go b/files_dedupe_test.go
index 87a51d1..6d24a85 100644
--- a/files_dedupe_test.go
+++ b/files_dedupe_test.go
@@ -64,6 +64,26 @@ func TestCrossFolderPrefersNonTrash(t *testing.T) {
 	}
 }
 
+func TestMergeProjectRootForDedupe(t *testing.T) {
+	old := &FileEntry{ID: jsonNum("1"), Title: strPtr("a.docx"), FileExst: strPtr(".docx")}
+	newer := &FileEntry{ID: jsonNum("2"), Title: strPtr("a.docx"), FileExst: strPtr(".docx")}
+	rootFiles := []*FileEntry{old, newer}
+	folders, byFolder := mergeProjectRootForDedupe("489", nil, nil, rootFiles)
+	if len(folders) != 1 || folders[0].ID.String() != "489" {
+		t.Fatalf("folders=%+v", folders)
+	}
+	if len(byFolder["489"]) != 2 {
+		t.Fatalf("root files=%d", len(byFolder["489"]))
+	}
+	groups := findWithinFolderDuplicates([]ProjectFolderFile{
+		{FolderID: "489", FolderTitle: "(project root)", File: old},
+		{FolderID: "489", FolderTitle: "(project root)", File: newer},
+	})
+	if len(groups) != 1 {
+		t.Fatalf("groups=%d", len(groups))
+	}
+}
+
 func TestIsTrashFolderTitle(t *testing.T) {
 	if !IsTrashFolderTitle("_trash-md") {
 		t.Fatal("expected trash")
diff --git a/files_stem.go b/files_stem.go
index b9552a6..02b2b53 100644
--- a/files_stem.go
+++ b/files_stem.go
@@ -3,10 +3,15 @@ package onlyoffice
 import (
 	"context"
 	"encoding/json"
+	"errors"
+	"fmt"
 	"path/filepath"
 	"strings"
 )
 
+// ErrFileExists is returned when --no-replace / no-clobber upload hits an existing stem|ext.
+var ErrFileExists = errors.New("onlyoffice: file already exists in folder (use replace or delete first)")
+
 // FileEntryStem returns the logical basename without duplicated extensions.
 // OO often stores title="foo.docx" and fileExst=".docx" (UI shows foo.docx.docx).
 func FileEntryStem(f *FileEntry) string {
@@ -116,6 +121,38 @@ func (c *Client) DeleteFilesByStem(ctx context.Context, folderID, stem string) (
 	return ids, nil
 }
 
+// AssertNoFileConflict reports ErrFileExists when localPath stem|ext is already in folderID.
+func (c *Client) AssertNoFileConflict(ctx context.Context, folderID, localPath string) error {
+	files, err := c.FolderFiles(ctx, folderID)
+	if err != nil {
+		return err
+	}
+	stem := UploadStemFromLocal(localPath)
+	ext := UploadExtFromLocal(localPath)
+	matches := FindFilesByDedupKey(files, stem, ext)
+	if len(matches) == 0 {
+		return nil
+	}
+	ids := make([]string, 0, len(matches))
+	for _, f := range matches {
+		ids = append(ids, fmt.Sprintf("%d", FileEntryNumericID(f)))
+	}
+	return fmt.Errorf("%w: %s%s in folder %s (existing file ids: %s)",
+		ErrFileExists, stem, ext, folderID, strings.Join(ids, ", "))
+}
+
+// UploadProjectFileNoClobber uploads only when stem|ext is not already in the project folder.
+func (c *Client) UploadProjectFileNoClobber(ctx context.Context, projectID, localPath string) (*FileEntry, error) {
+	folderID, err := c.projectFolderID(ctx, projectID)
+	if err != nil {
+		return nil, err
+	}
+	if err := c.AssertNoFileConflict(ctx, folderID, localPath); err != nil {
+		return nil, err
+	}
+	return c.UploadProjectFile(ctx, projectID, localPath)
+}
+
 // UploadToFolderReplacing deletes same stem+ext files then uploads localPath.
 func (c *Client) UploadToFolderReplacing(ctx context.Context, folderID, localPath string) (*FileEntry, []int, error) {
 	stem := UploadStemFromLocal(localPath)