"Already used" in User profile edit page. using Japanese
Using Jamroom
The profile_name_url needs to be in A-Z characters. Jamroom doesnt have a translitteration system to change 'マイケル' to 'michael' so what happens is the characters gets stripped and it becomes '' which is the same for anyone else using all japanese characters.
Try using the FORM DESIGNER on the profile update page to add the form field profile_name_url and enter a-z characters in there.
There is no issue with 2 profiles using the same profile_name but there is an issue with 2 profiles using the same profile_name_url because thats what gets used at yoursite.com/profile_name_url