When placing replicas using the replica placement policy BlockPlacementPolicyWithNodeGroup, the number of targets returned should be less than or equal to the number of node groups and no node group should get two replicas of the same block. The Junit test TestReplicationPolicyWithNodeGroup.testChooseMoreTargetsThanNodeGroups verifies this.
However, if the conf property "dfs.namenode.avoid.write.stale.datanode" is set to true, then block placement policy will return more targets than node groups when the number of replicas requested exceeds the number of node groups.
This can be seen by putting:
in the setup method for TestReplicationPolicyWithNodeGroup. This will cause testChooseMoreTargetsThanNodeGroups to fail.