Users' unique value are - Email address & username.
Unique means, that there cannot be the same values for email address or username.
Whenever there is a change in a user's active directory profile field, the SCIM send Qmarkets update request/s.
Before SCIM updates any user, it's checking if the user already exists in Qmarkets ('GET' request).
This check is done by searching for a user's unique identifier - which is usually username.
Example:
If Qmarkets get a 'GET' user SCIM request with username =%@test.com but no such user - then SCIM would try to creates this user as a NEW user in the system and will send a 'POST' request.
BUT - the new user cannot be created as its email address already exists in Qmarkets (and email address is a unique value, same as username).
To change system users' username values - please consult with your CSM.