Difference of GR with reference to Purchase order and With ref. to outbound delivery

Hi ,

Are there any differences when we post gr with reference to po and with reference yo delivery?

The scenario is that when we Post GR with reference to PO, we can perform the same MIGO GR to the outbound delivery of the same PO which could cause inconsistencies since it has alreay been posted. But when we post GR with reference to outbound delivery everything is ok.

Thanks in advance