Minor fix. [skip ci]

nomad/cli/client/migration.py

@@ -299,7 +299,7 @@ def missing(start_pid, uploads):
             for upload_information in upload_list:
                 source_upload_id = upload_information.get('source_upload_id', None)
                 if source_upload_id is not None:
-                    uploads.append(source_upload_id)
+                    uploads.add(source_upload_id)
 
         for source_upload_id in uploads:
             print(source_upload_id)