The big difference is that the imperfect is implied to be past-limited. Whatever is described is bound to an (at least implied) specific time or times. More accurately, if it’s in the imperfect, it is no longer happening or likely to happen again. The passé composé does not have that implication.

In the first case, there is a slight nuance between the two tenses. The imparfait would imply that it won’t happen again or at least didn’t happen for some time (i.e. “we would/used to see this often”). “On a déjà vu ça souvent” is exactly what I’d say about something I’m seeing right now (i.e. “I’ve seen this [often] before”).

In the second case, “je n’avais jamais la chance d’être sa copine” is restricted. It would refer to a specific set of occasions at which point the speaker wasn’t the (presumably) guy’s girlfriend. Furthermore, it is not contradictory with the idea the speaker was at one point the guy’s girlfriend. The passé composé makes a broader statement that being the girlfriend never occurred at any point.