I don't think it's a Relay issue. It's a Client one. Relays give Clients what they ask for. Nothing more, nothing less.
Clients are in control of when and how they contact relays. They can, for example, in theory, grow to know which people's posts are likely to be on local Relays, and get them from there, so that when they contact relays further away, they can say "posts by X, but NOT THESE ones, as we have them already".
