Hello Andreia,
It's an interesting challenge. It should be possible to do these checks via a microflow and create temporary objects where you capture this information for each account.
I think the fastest & most efficient would be to use OQL (View Entities)
https://docs.mendix.com/refguide10/oql/
Try the following
SELECT
u.Email AS Email,
u.FullName AS UserName,
COUNT(ur.id) AS NumberOfRoles
FROM Administration.Account AS u
INNER JOIN u/System.UserRoles/System.UserRole ur
GROUP BY u.FullName, u.Email
HAVING COUNT(ur.id) > 1
This query should give you what you asked for. Maybe you can try to improve the query to also display the roles list.