In the past, it was dead easy:
- Flag for moderator attention
- The Moderator will check to see if the two really are likely belonging to the same person.
- If it looks like it, the moderator will merge the users.
Only, that doesn't work anymore. A few weeks ago the Stack Exchange staff took away user-merge as a moderator power. Mistakes were made, and they require very fiddly database picking to undo. This gets really bad if a moderator doesn't exercise enough care and merges a 7000 rep old-timer into a 1-rep newly-registered user. Also, user-merges are SE-wide so the same accounts would be merged on StackOverflow, SuperUser, and anywhere else the two overlap.
User-merges are now a user-directed action. They have to:
- Go to the Contact Us link at the bottom of every page
- Select the Merge User Profiles option on that page
This will notify SE staff that a merge needs to be done, and they'll do it. To my knowledge, there is no way to notify SE staff on behalf of a user that such an action needs to be done.
So if you come across such things:
- Leave a comment pointing them to the user-merge link.
- If this is content that should be in the question, put it there yourself or mod-flag.
- If it's an answer to the question, also point out they can accept it themselves after their accounts are merged and 2 days have passed.