Why didn't Twilight built a clone army of the pinkie clones. First of all, she killed them( Clones or not they were living being with the fear of being dead) second, they could have a powerful unstoppable army.

Also, How did they could tell which one is the real pinkie by check who will stare longer at the paint drying? Pinkie said that only the real Pinkie will be focused, and will do something Pinkie won't do so she's the real one. But it's just prove which one has the strongest will.

Why didn't Twilight built a clone army of the pinkie clones.

Why do you expect that she would?

second, they could have a powerful unstoppable army.

Pinkie is not known for being perfectly reliable, and the Pinkie clones are even less dependable than she is. Even if they can't be stopped, they can probably be rendered ineffective.

But it's just prove which one has the strongest will

Yes, the episode's logic is pretty questionable here. It vaguely tries to imply that the strength of Pinkie's friendship proves that she's the real one, but never really justifies it.

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Yeah, but the idea of the mirror pool was used only once. Why? They could use it in a finale to bring a giant clone army of the main 6 attack of the clone style to fight someone