Add duplication guard in collab onChange to detect and auto-fix doubled content

Y.js CRDT merges can duplicate content when a client reconnects after
external file modifications (e.g., API PUT while collab is active).
The guard detects when content is exactly doubled and auto-trims it.

Bunker Admin

api/src/modules/docs/docs-collab.service.ts

@@ -257,8 +257,23 @@ const docsExtension: Extension = {
       return;
     }
 
-    // Write plaintext to disk (debounced by Hocuspocus's built-in debounce)
+    // Duplication guard: detect and fix content that appears to be doubled
     const content = yText.toString();
+    if (content.length > 20) {
+      const half = Math.floor(content.length / 2);
+      const firstHalf = content.substring(0, half);
+      const secondHalf = content.substring(half);
+      if (firstHalf === secondHalf) {
+        logger.warn(`Docs collab: detected duplicated content in ${documentName} (${content.length} chars), fixing`);
+        // Replace the Y.Text with just the first half
+        document.transact(() => {
+          yText.delete(half, content.length - half);
+        });
+        return; // The deletion triggers another onChange with the fixed content
+      }
+    }
+
+    // Write plaintext to disk (debounced by Hocuspocus's built-in debounce)
     try {
       await docsFilesService.writeFileContent(documentName, content);
       // Invalidate Redis file cache