happens to me with migrate CUCM from 8.6(2) to 12.5(1)
PCD&OLDCUCM&NEWCUCM seat on the same vlan
In my scenario it happens only when I've needed to be involved the migrate procedure while it PCD showed me error and stopped the process.
So when I wanted to start the process again, the new server and the old server was turned on in the same time and as I think it should cause to duplicate IP for the next step in PCD.
so if you have any error in the process of migrate and you want to retry the task, look on the ESXi and check if the old cluster on and the new is off, as it was before you start the task.