Shoutbox

I don't think this is an issue with Messenger, I think this is a relationship issue you need to settle with your girlfriend.

I was in the same position, was worried my girlfriend (now ex-girlfriend) would cheat on me, so I monitored her Messenger account, etc. Turned out she did, more than once. Your relationship isn't going to work out if you're constantly monitoring her Messenger account.

And I haven't heard of known issues about contacts being re-added due to multiple computer use, so yes, she's probably adding him back.