NIP: add idsOnly param to REQ? Relays would reply with event ids only, not the full event. You would only request the full event if you didn’t already receive it from other peers or local cache. This is basically how bitcoin advertises objects with inv messages which are then requested with getdata if needed.

Example:

1. Add idsOnly param to REQ messages, so the relay responds with event ids only, not the full event.

2. Get an event id that you don't already have. Request it and get the full event as response.

3. After a few hundred milliseconds, receive the same event id from 3 other relays. Don't request it this time, as you already have it. Up to 75% bandwidth saved, depending on the event content size in relation to the requests.