There are two reasons for duplicate events. One is client side, most clients send the same event to all the relays they're connected to. The other is if relays are syncing they accomplish the same thing via a sync regardless of the client efforts. If a client doesn't realize they're connected to two synchronized relays, they transmit twice as much and receive twice as much as they need to.

So what I'm saying is, in the future clients will get a lot smarter about relaying, only posting to the minimal set of relays to get a message out.. and relays will probably become more centered around communities of users where you *choose to be on a specific relay for specific content or reason.