[bugfix] dedup groupby columns in Deck visualizations (apache#4801)

When specifying the same column twice as a `Extra Data for JS` and
`Categorical Color`, an error is issued. This addresses this issue.

superset/viz.py

@@ -2036,6 +2036,7 @@ def query_obj(self):
         if fd.get('js_columns'):
             gb += fd.get('js_columns')
         metrics = self.get_metrics()
+        gb = list(set(gb))
         if metrics:
             d['groupby'] = gb
             d['metrics'] = self.get_metrics()