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Is it guranteed that the name will match with one role def only?

Name can match with multiple role definitions(edge case), but when deleting we ensure that if only 1 role is matched, then only delete, otherwise an error is thrown asking user to specify role by Id, as multiple roles with same name are found.
So yes, when deleting a role, name will match to 1 role def only, otherwise it will be an error.