XF 1.4 Import from vB Username Barbel == Bärbel

in XF are A = a and ö = o etc.
But this is wrong username Barbel is not the same user name Bärbel!
Nevertheless, says XF Username Bärbel existed (Barbel)

How can I make it clear that is not the same.
But this can not be true!
It's just the way the Ä = A, and a user should have the possibility, regardless of an existing user "Bärbel" call to "Barbel"!

Here must be found a solution!
Am I the first to this auffält or was it the other just do not care!
Because problems is indeed very extensive