Two slaves with the same server id will replicate successfully, but when they reach the end of the master's binary log, something freaks out and forces them to disconnect. This causes both slaves to reconnect, sync (no data needed), and have the connection die off quickly again. The result of this is rapid connection/disconnection by both slaves driving the load to 1+ on both slaves, and to around .3 on the master even in a completely idle system. This is bad. Therefore, server id collisions are bad.