tried to add entity (entity name) duplicate UUID (UUID string)

minecraft server 15w32b

this should not come up in the log and the code should not be executed in the first place

When i logged into a snapshot server in the log when i was loading new chunks it comes up with this
[(time) WARN]: Tried to add entity (entity name) with duplicate UUID 906ea7d8-f55f-4c1b-bc00-847abededcb3

Steps to Reproduce:
1. load up the server
2. log on to the server
3. load new chunks and check the log