In the past, it was dead easy:

1. Flag for moderator attention
2. The Moderator will check to see if the two really are likely belonging to the same person.
3. 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:

1. Go to the Contact Us link at the bottom of every page
2. 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.

In the past, it was dead easy:

1. Flag for moderator attention
2. The Moderator will check to see if the two really are likely belonging to the same person.
3. 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:

1. Go to the Contact Us link at the bottom of every page
2. 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.