What does "already dead" mean?

Question

In Spider-Man Into the Spider-Verse (2018) Alternate Universe Cut, after being teleported to an alternate universe, Peter B Parker, who finds himself half-naked, is arrested and placed in the back seat of a police car. The police officers inform him that Peter Parker, known as Spider-Man, is dead. They suggest he attend the funeral the following day:

Peter B Parker: It's a little hot back here. You mind cracking a window just a little?

Male Cop: Yeah, if it'll shut you up.

Peter Parker escapes from cops:

Peter Parker: Kills me to do this to you guys, but then again, I'm already dead!

What does "already dead" mean?

Answer 1

The police officers inform him that Peter Parker, known as Spider-Man, is dead.

Exactly; in this alternate universe, the original Peter Parker/Spider-Man belonging to it just died (we just witnessed along with Miles how he was killed by Kingpin).

Peter B. Parker, who is visiting this universe in which Miles Morales exists, 'jokes' that it doesn't matter he escapes police custody, because if he is technically the same person (sharing his name, moniker, and abilities), the cops would have arrested a dead person, anyway: "then again, I'm already dead".