no uniqueness enforcement on core_userrole table

Version
3.28.18

Describe the bug
The core_userrole table is not enforcing uniqueness constraints ...

galaxy=> select pulp_id,object_id,content_type_id,role_id,user_id,domain_id from core_userrole where object_id='3953';
               pulp_id                | object_id | content_type_id |               role_id                | user_id | domain_id 
--------------------------------------+-----------+-----------------+--------------------------------------+---------+-----------
 ed7f4879-9ce4-4fec-bc4e-9ba2f82ef089 | 3953      |              90 | b4b70856-51d1-4143-92f5-55cced716263 |    1737 | 
 018accc3-6934-70f7-bbbd-4150cf9b8610 | 3953      |              90 | b4b70856-51d1-4143-92f5-55cced716263 |    1737 | 
(2 rows)

To Reproduce
We use pulpcore.plugin.util.assign_role to set users as namespace owners. Our helper function is here ...

https://github.com/ansible/galaxy_ng/blob/master/galaxy_ng/app/utils/rbac.py#L63-L65

Theoretically, calling that multiple times would cause the duplicate entries but I have no confirmed.

Expected behavior
Only one row should be allowed for user_id + object_id + role_id (and maybe domain?)

Additional context
Originally found in galaxy.ansible.com's database

Start with pulpcore.plugin.util.assign_role, using the linked galaxy_ng/app/utils/rbac.py lines 63-65 as the caller, and inspect how rows are created in core_userrole. Confirm which fields define a duplicate, including whether domain_id belongs in the key. Done means repeated role assignment cannot create duplicate rows and the behavior is covered by appropriate database or application tests.