To reproduce

1. open two bibliographies
2. copy (Command-c) one (or more) entries from first bibliography
3. switch to second bibliography and paste with (Command-v)

Then in the entry is added to the second - but also duplicated in the first bibliography.

Info:

JabRef 5.0-dev--snapshot--2019-04-03--master--a273eb1fa
Mac OS X 10.14.3 x86_64 
Java 1.8.0_191

I think it might be linked with #4836 - also here pasting with right click does not trigger it.