New chunks delayed - log shows blizzard of <entity> with pending removal and duplicate UUID <uuid value>

Traveling by horse - reached limit of current chunk; chunk had not loaded.. on checking server log a large series of:
WARN]: Tried to add entity <type of entity> with pending removal and duplicate UUID <uuid> . Multiple entity types and UUID values. new chunck doesnt load. Restarting the server results in immediate crash. Re-restarting successful, chunk is present on reconnecting.