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 Username. To make sure that both of you can use all Wikimedia projects in future, we have reserved the name Username~mediawikiwiki 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.
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.
Community Liaison, Wikimedia Foundation