fix graph bug about second retrieval (#1688)

### What problem does this PR solve?

#1651

### Type of change

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

graph/component/base.py

             break
 
         if upstream_outs:
-            return pd.concat(upstream_outs, ignore_index=False)
+            df = pd.concat(upstream_outs, ignore_index=True)
+            if "content" in df:
+                df = df.drop_duplicates(subset=['content']).reset_index(drop=True)
+            return df
         return pd.DataFrame()
 
     def get_stream_input(self):