Can Piercing Light target the same unit or must they be two different units?

Piercing Light must target two different units if you choose to use both parts of the effect.

The card text states: "Deal 2 to a unit at a battlefield, then deal 2 to up to one other unit."

Because the second part of the instruction specifies an "other" unit, you cannot select the same unit that was targeted by the first instance of damage.

Key Details:

• First Target: Must be a unit at a battlefield.
• Second Target: Can be any unit (at a battlefield or in a base) as long as it is not the first unit chosen.
• Optionality: The phrase "up to one" means you can choose to deal the second instance of damage to zero units if you wish, but you still cannot choose the first unit again.
• Repeat: If you pay the [Repeat] cost, you perform the entire instruction again. For the repeated instance, you may choose the same two units again, or two entirely different units, because the "other" restriction only applies within a single resolution of the spell's text.