Hmmm, you may be right. I boldly assumed that Stack Exchange was also using Akismet (which I do not actually know). I think, but will need to find examples, that I also saw the review being triggered for existing posts, after it was duplicated to Stack Exchange. Most often it seems the Stack Exchange post was older, which also explains copy-paste triggering the “typed their first post suspiciously fast” review.
Also, for some time, we surely saw many false positives after posts were edited. This made me assume the filter was confused by its own algorithm to find duplicates, not understanding the duplicate from some online database was the very same post on the very same forum. When searching for the cause of this, I did not find any references in Akismet’s services.
So, many assumptions. I’ll try to find some examples, but maybe even more posts are duplicated between the forum and other places, and maybe I’ve only found few of them after all. 