Crash - Exception ticking world -> Entity is already tracked!

Crash has occurred four times in two days, and only at high (>200) Y-values. I am trying to build an iron golem farm. Upon crash all of my data is saved and I do not lose progress. I seem to be unable to find a duplicate issue or workaround for this.

I have not entered The End yet; that issue is unrelated to mine though the error message is similar.

Thanks for reading. Good luck.