User can change display name to existing one even if Site settings require unique display names

Steps to reproduce:

1. Create a first user

   • UserName: user1

   • DisplayName: MyName

2. Create a second user

   • UserName user2

   • DisplayName: MyName

3. Go to Site Settings and activate "Require Unique Display Names"

4. Change user2 DisplayName to "OtherName"

5. Logout and relogin with user2

6. Go to profile and change the DisplayName back to "MyName"

What happens: it works

What should happen: an error message should show that says this DisplayName is already taken.