For Existential Dread, can you use it on different units if you repeat costs?

Yes, you can target different units (or the same unit) for each execution when you pay the Repeat cost for Existential Dread.

According to Rule 820.2.a, when you use the Repeat keyword, choices for the additional execution do not have to be the same as the choices made for the initial execution. This means that when you play Existential Dread and pay its Repeat cost, you are effectively performing the effect twice as two separate instances, allowing you to choose a new target for the second execution.

Key Considerations:

• Targeting: Because Existential Dread chooses a unit ("Stun an attacking enemy unit"), each instance of the effect targets a unit. If you choose two different attacking enemy units, you are targeting two different units.
• Deflect: If either (or both) of the units you target has the Deflect keyword, you must pay the Deflect cost for each instance of targeting (see RiftJudge FAQ #9189).
• Requirements: Remember that Existential Dread can only target an attacking enemy unit. Since you are making these choices when you play the card, both targets must be valid "attacking enemy units" at the time of finalization to be legal targets for the respective instances of the effect.

This interpretation is based on the Repeat keyword rules (Rule 820.2.a) and established mechanics regarding targeting and repeated effects as documented in RiftJudge FAQ #8461 and #9078.