Welcome to the Q&A Forum

I have this situation on my sites too. This might not be right, but to double check that my 301s aren't being seen as duplicates of the 200s that they are redirecting to I'll filter my crawl test tool report from Moz to only show the records where the HTTP status code doesn't equal 200 and duplicate page content is yes. I have a couple different crawlers that I use and they each work a little differently. I think some might look at your situation and return page A as a 200 just because when it resolves to page B it is a 200, where other crawlers are more sensitive and see page A as a 301. I hope that helps!