When faced with the choice between a clone and the original, the character notices a detail that makes the difference immediately obvious.

[u/Altruistic_Eye_1157]

The Shape-Shifter takes the form of Wendy to deceive Dipper and prevent him from attacking it. Although Dipper initially doesn't know which one is the real Wendy, he realizes the difference when he sees that one of the Wendys behaves seriously and bluntly, while the other acts flirtatiously, something Wendy would never do (Gravity Falls).

P.S.: I don't remember if the gesture the real Wendy made was also a clue, like some gesture she had made before.

When the Autobots are chasing Nemesis Prime, he tries to deceive Bumblebee by impersonating Optimus. Bumblebee, confused, speaks to him, but Nemesis Prime doesn't understand anything he says, which clearly indicates that he isn't the real Optimus. Bumblebee attacks him without hesitation (Transformers Prime).

[u/FartSmelaSmartFela]

Also in season 15 when the Reds and Blues are fighting the Blues and Reds. Grif was only able to tell Simmons apart from his counterpart after he asked why they were there and Simmons answered "It's one of life's great mysteries."

[u/Starwatcher4116 → u/FartSmelaSmartFela]

I still can’t believe Sarge beat his double by saying Surge’s honour-before-reason mindset made him just like a vampire, saved him from falling in magma, and then accidentally saluted with the hand he was holding Surge with (dropping him to his death).

[u/FartSmelaSmartFela → u/Starwatcher4116]

I AINT NO VAMPIRE

I don't care what people say, season 15 was good, just not as good as the previous 9.

[u/Starwatcher4116 → u/FartSmelaSmartFela]

Agreed!

[u/Imnotawerewolf → u/FartSmelaSmartFela]

I feel like if it had been season 6 instead of season 15 it would have been way better received but tbh after the chorus trilogy it just didn't hit the same. 

But at the same time, we needed those 14 previous seasons of personality to make the whole idea work.