I recently finished my Phd in pure maths and I am looking for open problems in my research area, functional analysis. Without going into the details, I stumbled onto an interesting problem and I shared it with one of my collaborators who proved a nice result. After a google search, however, I found a 2013 paper on the archive where the author considered the same problem and basically solved it. At first, I was a bit perturbed about this but it seems to happens quite often; the prime number theorem is one example of this phenomena.
I would like to know if this has happened to anyone in the math overflow community and how they dealt with it.