I had someone do this once. They wrote their fic and commented on mine that they had already posted it. I wasn't overly offended, though my beta reader was kind of pissed because the person lifted entire scenes from my fic and basically retold my own story, just in their words.

I've also been on the other side of this. I did a complete rewrite of a fic idea, but I will never post it because I feel like it would be very disrespectful to the original author. I rewrote it so I can enjoy reading it. I loved the idea that the original author had, but they seemed quite young and their grammar and vocabulary was reflective of that. It wasn't a writing style that appealed to me.