Not sure how reproduceable this bug is, but today (using latest sources), I double clicked on a contact and it lagged (nothing happened, but the contact list sort of froze). I wasn't sure if I correctly clicked, so I double clicked again. It was still lagged, but finally opened the window...with two tabs for the same user.
I'm guessing it allowed opening the tab the second time because it hadn't opened the tab by the time I double-clicked the second time.
So, maybe there shoudl be some check to verify a tab hasn't already been requested before you allow a second request. Not sure how feasible that is.