An attributed relationship class uses a cross reference table to relate objects from two tables. In this table, each row holds the foreign keys to the two objects. There is no integrity check to make sure there is only one row for each unique combination of foreign keys. In other words, there can be two rows in the cross reference table with the exact same foreign keys.
This is currently by design.
The ability to relate the same two objects together multiple times exists. This is especially important when other attributes are stored on the cross reference table.