When a new user account shows up, and it looks very similar to an existing user account, it would be nice to leave a comment saying something like "Are you this other user? If so, here's what you should do."
Hi [insert username]. Your name looks familiar; is [this other account](link to other account) also yours? If so, please [ask for them to be merged](/help/merging-accounts). That way, you can keep track of all of your activity here in one place.