Looks like we're starting to get fake accounts like Jack@primal.net on Nostr, but that's defrayed by the trust gained by Jack's main account. A sophisticated attacker might spawn accounts that aren't on the platform already, making things difficult to track. Right now, Nostr's verification process is longest-standing npub with most followers, but perhaps, in time, there should be tracking on people who zap back and forth as "verification". This has the incidental benefit of getting people to use Bitcoin/LN even if they're not Bitcoiners.
Discussion
That is Jack’s main account. NIP-05s and WoT help determine fakes. Not all clients make it obvious yet though.
I'm seeing two accounts if you search for jack@primal.net, I'll edit and link though. 
That’s weird cause it’s not possible for 2 people to have the same nip-05. What client is that?
The Nostr Address isn't validating for them, as the hex pubkey is not correct. NIP-05 should also be handled as case-insensitive. So both `Jack` and `jack` and `JACK` will all be valid for the same pubkey. Which appears to be the case if you use any of those with this https://primal.net/.well-known/nostr.json?name=jack it shows the same pubkey. That doesn't stop someone from adding it as their Nostr Address, but it won't validate.
that seems like a bug in the nostr client. it shouldn't display the accounts with invalid nip-05s when searching by nip-05.
Primal
That’s funny cause it’s a primal nip-05 😂
weird no users show up when i search that jack@primal.net in primal. i just checked jack's kind 0 and he doesn't even have a nip-05 set up on his profile. jack@primal.net is his lightning address though.
Yeah I think WoT works for established people on the network, but not sure what stops people from registering more plausible IDs (say Twitch streamers?) who aren't currently on Nostr