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On cards like Duress, why does it ask me to select an opponent?

You can't even target yourself like you could with Thoughtseize.

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6 months ago - /u/WotC_Jay - Direct link

Originally posted by fractalspire

You might be asking this because you've noticed that some cards will auto-target (for example, A-Blood Artist automatically targets your opponent). If so, the difference here is that Arena will automatically pick the only legal target for abilities (so, Conclave Tribunal will automatically target the only nonland permanent your opponent controls, for example) but will not automatically pick targets when you're actually casting a spell. Probably this is intended as misclick protection by making you actually confirm all targets when casting.

This is exactly right. When casting a spell, we like to make sure there's at least one choice you're making, so that you get an "Oh wait, what? No! Cancel!' window