diff --git a/tools/dataset.py b/tools/dataset.py
index 34af52ad..8a82f713 100644
--- a/tools/dataset.py
+++ b/tools/dataset.py
@@ -83,7 +83,7 @@ def _make_group(cg, chain_group_list):
             new_len += cg_len
             new_group += chain_group_list[i]
             new_added = True
-      new_group_str = ','.join(sorted(new_group))
+      new_group_str = ','.join(sorted(set(new_group)))
       if new_group_str not in chain_seen:
         yield new_group
         chain_seen.add(new_group_str)