This is where i think petnames work well. If you petname the real McCoy, and your client shows your petname in a special font or color, you can have it say "(the real) Jack" and not be fooled by other "Jack"s. (I keep meaning to add this to gossip, it is a small and easy change, but I keep diving deep into the hard stuff first).

Of course initially this doesn't help if you don't know which keypair is the real Jack. That probably has to be discovered out of band though (e.g. professing your keypair on Twitter).

Maybe 1984-aware clients could flag as "possible impostor" any account that other people have reported as being such.