Please read the message carefully. A user's "Real Name" is not allowed to match another user's "Username". This prevents a user from masquering as another.

For example, consider a user "jdoe" with a real name of "John Doe". A new user with user name "fsmith" cannot set their real name to "jdoe", but "John Doe" is acceptable.

Yes, I can see the reason behind avoiding masquerading, but my issue in 0005949 is that I used my back button and was not able to correct the error. So I guess the anti-caching headers need to change to avoid the browser reloading the page on "Back", or a ticket needs to be allowed to be re-used.

So it is ok, if you want to prevent shadowing other users, but still it doesn`t work...how you explained.
So here is my way to the error:

I create the following user...
Real Name: Test Tester
Login: Bubbel

Now I want to create another:
Real Name: Test Tester
Login: Lupo

The Login is accepted but not the change to the realname.

The check is wrong if you create a new user from the "manage" menu. It's missing if you update a user from the "manage" menu.

Fixed in CVS:
core/user_api.php -> 1.105
manage_user_update.php -> 1.37