So I run 2 different versions of SSMS (Have done for a couple of years), currently SSMS 21 & 22. my question and issue is this. If I have say 8 tabs open in SSMS 21 and close it, it saves them (great feature) BUT if I then open SSMS 22 it will reopens those tabs from SSMS 21 in 22. and if I have both 21 & 22 open and close them both around the same time and reopen 21 or 22 all of the tabs from both combined version open in a single SSMS instance. 

See this support ticket for more info.
http://productsupport.red-gate.com/hc/requests/346200