Got this annoying bug, S4B is saving a missed conversation to my Inbox, same duplicated conversation 300 times and counting:

Deleting the files in this folder seems to help.

%LOCALAPPDATA%\Microsoft\Office\16.0\Lync\{sip_id}\History Spooler\

Thanks to this thread:

https://community.skype.com/t5/Using-Skype-in-your-business/multiple-duplicate-messages/td-p/4299831