If Fiora, Peerless attacks a single unit but the defender reveals a hidden Teemo in reaction to the attack trigger, does Fiora's might still double when the trigger resolves?

Ruling: Yes, Fiora's might still doubles. The trigger condition is only checked when determining whether the trigger should go onto the chain, not when it resolves.

Sequence:

• Fiora attacks what appears to be a one-on-one situation
• Attack trigger checks condition and goes onto the chain as "double my might this combat"
• Defender reveals hidden Teemo
• Trigger resolves and doubles Fiora's might regardless of the changed battlefield state

Nuances:

• This works this way because Fiora uses "when" (trigger) rather than "while" (passive). A passive ability like "my might is doubled while I am attacking one on one" would continuously check the condition and would not double might after Teemo is revealed.
• The comma placement in the card text determines what is the trigger condition versus what is the effect.