Deduplication is defined as bulk action on user role agenda. For admin documentation see Roles assignment deduplication.
For comparing is used method isDuplicit in IdmIdentityRoleService. This method is called directly for each manually added role for specific contract.
Role in parameter name ONE will be removed if method return true (only in deduplication bulk action). Method isDuplicit is also called for example from reports.
Whole method can be overridden and changed the behavior as your project need.
Also concepts IdmConceptRoleRequestDto can be checked for duplicities. The methods removeDuplicities and markDuplicities in service IdmRoleRequestService can be used for mark and remove these duplicities in concepts and assigned roles. The methods are nov available only for your implementation in core is never used.
In future method markDuplicities will be used for mark duplicated concepts in frontend agenda.