A little earlier...
Before Shinji ran into the fake Hidan, Hidan and Tobi had each already encountered similar situations.
Tobi was suspicious by nature and found it hard to trust anyone.
So the moment Shinji and Hidan appeared, he activated the Mark's sensory ability.
Unfortunately, the impostors had no such function.
Tobi therefore didn't even bother speaking and simply blasted both of them apart.
Hidan's experience was rather rougher. His brain didn't work very well, after all.
He completely forgot that the Mark had a sensory ability and let Fake Shinji run circles around him. Only when a blade pierced his heart did he finally catch on.
Then he successfully turned the tables and killed Fake Shinji.
That should have been the end of it.
But with some deliberate prompting from his opponent, Hidan went on to blather about all sorts of things, even exposing his immortality.
Of course, it was also thanks to him being a genuine idiot that he never remembered anything about the Mark and thus didn't casually spill that important piece of intelligence as well.
Naturally, he also managed to mislead his opponent, allowing Shinji to fool him.
"Shinji, I finally found you! Thank goodness."
"Yeah, thank goodness."
Shinji walked up and killed "Tobi" with a single slash.
"Stop! Are you real or fake? When did we first meet?"
"April first. April Fools' Day."
The Kubikiribōchō split "Hidan" vertically in two.
"Hidan's in danger! Come with me!"
"Sure thing!"
Shinji grabbed "Tobi" by the head, slammed him into the ground, and ground his head to pieces.
He walked and killed, walked and killed, until even he had trouble keeping track of how many "Hidans" and "Tobis" he had taken out.
Clearly, after the first attempt, this tactic was unlikely to work again.
But his opponent's