It's the latter. The sentence means "I've seen (scenes like this) many times". It can have an accusatory overtone, so something like "more than enough times" may be better.
散々 rarely means "(spatially) dispersed" in modern Japanese, and it never means "occasionally/rarely".
Forgetting what you have learned is a bad thing.
So the scene is that G1 is happy to know that MC did the same (bad) thing when he was a student. MC tells G1 that it is not something she(?) should be happy about, and 俺も俺だけど means something like "well, I did wrong, too".
The intention of the whole phrase is "The fact I did the same thing should ...