This is how we do it.
Your account will be renamed edit
Hello,
The developer team at Wikimedia is making some changes to how accounts work, as part of our on-going efforts to provide new and better tools for our users like cross-wiki notifications. These changes will mean you have the same account name everywhere. This will let us give you new features that will help you edit and discuss better, and allow more flexible user permissions for tools. One of the side-effects of this is that user accounts will now have to be unique across all 900 Wikimedia wikis. See the announcement for more information.
Unfortunately, your account clashes with another account also called Crabier. To make sure that both of you can use all Wikimedia projects in future, we have reserved the name Crabier~enwiki that only you will have. If you like it, you don't have to do anything. If you do not like it, you can pick out a different name. If you think you might own all of the accounts with this name and this message is in error, please visit Special:MergeAccount to check and attach all of your accounts to prevent them from being renamed.
Your account will still work as before, and you will be credited for all your edits made so far, but you will have to use the new account name when you log in.
Sorry for the inconvenience.
Yours,
Keegan Peterzell
Community Liaison, Wikimedia Foundation
23:10, 19 March 2015 (UTC)
Renamed edit
This account has been renamed as part of single-user login finalisation. If you own this account you can log in using your previous username and password for more information. If you do not like this account's new name, you can choose your own using this form after logging in: Special:GlobalRenameRequest. -- Keegan (WMF) (talk)
11:46, 22 April 2015 (UTC)