If Vision picks a power on his turn and then gets knocked out of Dense as a Diamond but ends back on the power. Does he get the chosen power again?

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As Dense as a Diamond or Lighter han Air.: At the beginning of your turn you may choose: Invincible or both Shape Change and Super Senses. Vision can use the chose powers until you choose again.

As long as he is knocked from one click with the power to another, the power is never lost, so the duration persists.

If he is damaged onto a click without the power, the power is lost and so the duration expires and he could not use the powers until he chooses again, even if he is damaged onto another click with the power.