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 is placed first but resolves last.

When your Diana moves into the enemy Diana's battlefield, combat opens and both "When a showdown begins here" triggers fire simultaneously in Step 1 of the Combat Showdown (Rule 464.2.b). Both triggers are then placed on the Combat Chain with the attacking player first, then the defending player (Rule 464.2.e.1 — same ordering as attack/defend triggers, FAQ #3574). Since chains resolve last-in-first-out, the defending Diana's trigger sits on top and resolves first — her controller gets first crack at paying [1] to Predict and draw a spell — then the attacking Diana's trigger resolves. This is the same principle as Kha'Zix vs. Diana: chain position, not card identity, decides the order.