No, it's not a duplicate. All of our connections were good. /list shows the duplicates AND if the person reconnects during what happens with us they CAN'T move around like normal, they just get kicked again, making s second/third/fourth duplicate. Once they all do leave, the player still cannot play normally. The ONLY fix we have been able to do was change the physical machine the server is running on.
The issue isn't the player duplication in its self, it's that THE ENTIRE SERVER becomes unplayable for EVERYONE and ONLY switching the machine it is hosted on fixes the issue for that world.