I admit that I really like the idea of adding "Duplicates" as a Scene Aspect. But I think you're right - for the intended effect, a Block is the better approach.
Here's another question: if you are using a special illusion Block (i.e. a Block against someone's ability to perceive/target you), does the Block disappear if someone's Alertness beats it?
Mechanically, magical Blocks dissolve once they are beaten. But since Veils are special-case blocks, I wasn't sure if they followed different rules for this as well. (Just because an enemy locates you once, it doesn't mean that his friends can see you, and it doesn't mean that he'll be able to find you on the next exchange if you move.)