If Yasuo attacks and triggers a showdown, then during the showdown you play Flash to return him to base and then play Zenith Blade to move him back to the battlefield, does Yasuo get a second 'When I attack' trigger before damage is dealt?

Ruling: Yes, Yasuo would get a second "When I attack" trigger. When Yasuo returns to base via Flash, he stops being an attacker. When he re-enters the battlefield via Zenith Blade during the same showdown, he becomes an attacker again and triggers his attack ability a second time. Combat damage hasn't been dealt yet since the showdown hasn't ended.

Sequence:

• Play Flash first (Yasuo must leave the battlefield before Zenith Blade can target him)
• Let Flash resolve, returning Yasuo to base
• Play Zenith Blade to move Yasuo back to the battlefield
• Yasuo's "When I attack" trigger goes on the chain again
• The trigger resolves before the Combat Damage Step

Nuances:

• This interaction is based on Rules As Intended, not Rules As Written. Attack triggers joining combat after the initial chain is not officially in the rules yet but will be added.
• You cannot play Zenith Blade first because Yasuo would be an illegal target while still on the battlefield.
• If Yasuo is killed by a reaction/action spell before his effect resolves, the effect checks his might, sees he's dead, and deals 0 damage.
• The Combat Damage Step only happens once the showdown ends, so units killed during the showdown won't be accounted for during damage calculation.