Do you need to pay Deflect cost twice on Falling Star?

Yes, you must pay the Deflect cost twice if both instances of damage from Falling Star target the same unit with Deflect.

According to Rule 809.1.c, the Deflect keyword functions as a mandatory additional cost for spells and abilities that "choose" a permanent with Deflect "for each time they choose me." Because Falling Star contains two separate instances of "Deal 3 to a unit," it technically chooses (targets) a unit twice if you decide to select the same unit for both instances.

Key Details:

• Per-Targeting Cost: Deflect is not a one-time cost per card played; it is a mandatory additional cost for every individual instance of targeting.
• Timing: These costs are mandatory and must be paid when you play the card during the "Pay Costs" step of the chain finalization (Rule 353.2.a.2).
• Total Cost: If you target a unit with Deflect 2 with both instances of Falling Star, you must pay 2 Power for the first target and 2 Power for the second, totaling 4 Power in Deflect costs.

This interaction is confirmed by RiftJudge FAQ #9907 and FAQ #5107, which explicitly state that for multi-target spells like Falling Star, the Deflect cost applies to each specific time the unit is chosen.