Reference:UserMerge

Extension: UserMerge

Overview
Description:	Merges several users into one account.
State:	stable	Dependency:	MediaWiki
Developer:	Tim Laqua, Thomas Gries, Siebrand Mazeland	License:	GPL v3 only
Type:	MediaWiki	Category:	Administration
Edition:	BlueSpice free, BlueSpice pro, BlueSpice Farm, BlueSpice Cloud
For more info, visit Mediawiki.

Features

UserMerge extension allows wiki users with the usermerge permission (Bureaucrat by default) to merge one Wiki user's account with another Wiki user's account.

The extension allows to

• merge (refer contributions, texts, watchlists, edit count of a first account A to a second account B
• delete the first account A after the merge (option box)

Limitations

• you cannot delete a user A without having merged the user A to B
• you cannot merge your own account (logged-in user) into a different user

Extras

• if you omit the "New User" field, then the extension auto-populates the New User as "Anonymous" (user_id 0), and ask you to confirm a merge to Anonymous. This is used for user deletion: you first empty (merge to user 0) contributions of a user A, and then delete the user A.
• If your version of the extension complains about an empty "New User" field you can fill in  "Anonymous".

For more information please visit MediaWiki.