Comment by bbarnett

Both Microsoft and Google seem to do it just fine

I routinely get between 5 and 10 duplicates of DMARC reports from Google for gmail. Searching on this, it's a known phenomenon. No one else has this issue. I don't get dupes from outlook.com, hotmail, yahoo, etc, etc.

But it's Google. Fairly typical for their modern quality control and engineering practices. I view them as a little like when Volkswagen stopped being run by engineers, and instead by accountants.