diff --git a/gnommo/preprocessor.py b/gnommo/preprocessor.py
index 568ccd9..09c9662 100644
--- a/gnommo/preprocessor.py
+++ b/gnommo/preprocessor.py
@@ -662,11 +662,15 @@ def preprocess_video(
         # No filters defined, return original file
         return videos_dir / video_source.source_file
 
-    # Use custom intermediate dir if provided, otherwise default to videos_dir/intermediate
+    # Use custom intermediate dir if provided, otherwise default to videos_dir/intermediate.
+    # Always namespace by video_id so segments processed in parallel (or across
+    # overlapping runs) never share the chunks/ dir and concat.txt — otherwise
+    # they overwrite each other's chunk_NNNN.mov and one segment's footage bleeds
+    # into another's processed output (duplicated/inflated narration).
     if custom_gnommo_scratch:
         gnommo_scratch = custom_gnommo_scratch / video_id
     else:
-        gnommo_scratch = videos_dir / "intermediate"
+        gnommo_scratch = videos_dir / "intermediate" / video_id
     gnommo_scratch.mkdir(parents=True, exist_ok=True)
 
     # Start with the source file (relative to videos_dir)