The message is stored in the language files.
You can have it display whatever you wish.
GOTO CP
Scroll down to Languages and select it.
Select the language editor tab.
On select specific language file select generic.php
Click on go
Scroll down till you see a key name with: LOGIN_EXISTS
Edit the message to whatever you like.
then save.
This is not really a bug though.
I don't quite get the second part of your message but it sounds like you are asking for a feature request.
Which also is not a bug.
