diff --git a/mattermost-dl.py b/mattermost-dl.py
index 54f736f..c75d39b 100644
--- a/mattermost-dl.py
+++ b/mattermost-dl.py
@@ -326,7 +326,12 @@ def export_channel(d: Driver, channel: str, user_id_to_name: Dict[str, str], out
     print("Exporting channel", channel_name)
     after_ts = _parse_filter_timestamp(after) if after else None
     before_ts = _parse_filter_timestamp(before) if before else None
-    if after:
+    filtered_channel_name = ''.join(filter(lambda ch: ch not in "?!/\\.;:*\"<>|", channel_name))
+    output_filepath = pathlib.Path(output_base) / channel_name / (filtered_channel_name + ".json")
+    if merge and after_ts is not None and not output_filepath.exists():
+        print("  no existing export for this channel, fetching full history")
+        after_ts = None
+    elif after:
         print(f"  fetching posts after {after}")
 
     # Get all posts for selected channel
@@ -416,7 +421,6 @@ def export_channel(d: Driver, channel: str, user_id_to_name: Dict[str, str], out
             simple_post["files"] = filenames
         simple_posts.append(simple_post)
 
-    filtered_channel_name = ''.join(filter(lambda ch: ch not in "?!/\\.;:*\"<>|", channel_name))
     output_filename = filtered_channel_name + ".json"
     output_filepath = output_base / output_filename