If Diana attacks a BF with another Diana, which Diana their showdown trigger first. Diana lunari

The defending Diana's trigger resolves first — the attacking Diana's trigger is placed on the chain first, so it resolves last.

When combat opens, both "when a showdown begins here" triggers become pending (Rule 464.2.b), before attacker/defender designations exist. They're then put on the Combat Chain with the attacker's pending triggers first and the defender's last (Rule 464.2.e.1). Since chains resolve last-in, first-out, the defending player's Diana Predicts first, then the attacking player's Diana — after any attack/defend triggers in between.