I often doubt my OCD diagnosis though apparently it's quite clear that I have it. I'm diagnosed with it, my therapist is sure I have it, it causes me quite a lot of problems (though perhaps not the "classic" problems one tends to think of when thinking of OCD) etc. Last time my OCD caused a panic attack I knew that the anxiety was caused by OCD and that it made me re-check things for a couple of hours. But then, just a few moments after the panic attack was over, I started doubting my diagnosis again and I even started to think that maybe I had induced the panic attack myself. Intentionally. That it wasn't OCD and that I was probably misdiagnosed. I reckon that must be OCD too.

So yes, I think that some people might doubt their diagnosis even though the diagnosis is correct. Also, someone once told me that "doubting your OCD diagnosis is OCD at work".