Do not move a shard to a node that it already exists on

src/backend/distributed/operations/shard_rebalancer.c

@@ -2795,10 +2795,18 @@ FindAllowedTargetFillState(RebalanceState *state, uint64 shardId)
 				shardId,
 				targetFillState->node,
 				state->functions->context))
+		{
+			bool targetHasShard = PlacementsHashFind(state->placementsHash,
+													 shardId,
+													 targetFillState->node);
+
+			/* skip if the shard is already placed on the target node */
+			if (!targetHasShard)
 			{
 				return targetFillState;
 			}
 		}
+	}
 	return NULL;
 }