Comorbidity is quite common in people with BP (not necessarily for ADHD, but other psychiatric disorders too). I don't have stats handy. Still, as another poster mentioned, BP and ADHD symptoms can overlap.

I was diagnosed with mild ADHD first, and then BP2. Not being treated for the ADHD, as BP is the more serious concern. It's possible that many people with BP2 are misdiagnosed with ADHD because of some shared symptoms (also, depression is common in people with ADHD).

Sometimes I doubt my ADHD diagnosis, although it's not something that I think about often. My aim right now is to make my BP as manageable as possible.