Username of existing user should not be displayed

I was trying to create a new user but it happened that email I have provided was already used by another user. Username of that existing user was displayed in the error message. This seems wrong to me, FishEye instance can be configured to hide the visibility of user list and their emails but this seems to be not honoured here.
Also, can't see a real value of displaying name of existing user, simple message stating such an email address is already used by another user would do.

See