Fix duplicated communitiy (#4187)

### What problem does this PR solve?

#4180

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

graphrag/leiden.py

         if "communities" not in graph.nodes[n]:
             graph.nodes[n]["communities"] = []
         graph.nodes[n]["communities"].append(community_title)
+        graph.nodes[n]["communities"] = list(set(graph.nodes[n]["communities"]))