It is a mess for the user if a moderator merges two accounts if he/she suspects that they are the same person. Especially for those accounts that are created for example using their Gmail, OpenID etc account. If the moderator gets things wrong it can lead to a massive problem.
This doesn't take into account:
- Possible breaches in the users' privacy. All merged users will gain access to any non-public information provided by the main account, such as real name, email address, and/or OpenID
- Legal actions and acceptance of responsibility (who is the one to blame and to pay?)
- Legal conflicts with other jurisdictions for which the T&C of the site may be found out of order or non applicable
I think the safe move is to delete one of the accounts or both but merging accounts will lead to problems if somebody get things wrong.