Can someone else publish a paper based on independently achieving a result that is in a preprint of mine?
Do you know that they were aware of the preprint? Scoops are bound to be common if many groups are working on the same problem or starting to flesh out a new area with lots of low-hanging fruit. If they were aware of your preprint, and you can prove it, they should have cited your preprint at the very least, and you might get them to publish an errata with the citation. But, if they didn't know about it, they got there first, and you'll just have to accept that.