I use Falling Star against enemy Soaring Scout with Guardian Angel, does the unit die twice?

To answer your question: No, the unit does not "die twice."

When you play Falling Star targeting a Soaring Scout equipped with Guardian Angel twice, the interaction proceeds as follows:

The Sequence

1. Playing Falling Star: You play Falling Star and declare both instances of damage targeting the same Soaring Scout. The spell is placed on the chain.
2. Resolution: Falling Star resolves, dealing 3 damage twice.
3. Damage and Replacement:
   • The Soaring Scout takes 6 total damage.
   • Because the unit takes lethal damage, Guardian Angel triggers as a replacement effect. It replaces the "death" event: the Guardian Angel is destroyed, and the Soaring Scout is healed, exhausted, and recalled to your base.

Why it doesn't "die twice"

• The Event: The death was prevented by the Guardian Angel replacement effect. The unit never actually "died" in the game's rules; the event was replaced by the gear's effect.
• Conclusion: Because the death was prevented, the unit does not die.

Note: I could not fully verify this ruling against the retrieved rules.