The more I think about the latest episode the more disturbed I'm at the possibility of the original Pinkie being murdered already.
Also, apology to Razek but I am starting to dislike Twilight a little this season, who apparently turned a frog and a bird into oranges without fixing them up, ruthlessly killed all the Pinkie clones and is practicing dark magic.
The original Pinkie wasn't killed. Or rather, "sent back", because whatever form of existence the copy-Pinkies were, the episode made it clear that they weren't being "killed", they were being returned from whence they came.
Twilight's book made it clear that it was possible to "send back" the wrong Pinkie, and that such a thing would probably be undesirable, so there's some risk in sorting out the real Pinkie from the copies.
But the copy Pinkies didn't have the real Pinkie's knowledge and memories, just a vague copy of her relentless desire to have fun. Pinkie had to teach them the names of her friends using pictures, and the first copy-Pinkie couldn't even get the names right (she didn't seem to care what their names were, this was something Pinkie was forcing on them, telling them that they need to know it because Pinkie says so), and she didn't know what or where "Ponyville" was. They were just a bouncing pink swarm, blurting the word "fun". Even the first copy kept doing that. They were never true copies.
The copy-Pinkies had zero regard for the feelings of Pinkie's friends, like how they ruined Rainbow Dash's relaxation time in their mindless pursuit of fun, and wrecked Applejack's barn raising. The original Pinkie went out of her way to find a raft for RD to take a nap in, that way Pinkie could swim and push the raft while RD slept in it, and it would be a shared fun/relaxation experience. The copy-Pinkies showed no such consideration. The consumption of "fun" was their only goal.
But Pinkie Pie fell into despair, because she didn't recognize any real difference between herself and the copies, at which point she seriously questioned her own existence and accepted her possible nonexistence. There was no broken emo-copy. The one acting sad really was the one and only real Pinkie.
The real Pinkie missed her friends, and -having seen how uncaring the mass of Pinkies were towards her friends- concluded that the one Pinkie who loved her friends more than anyone must be the real one (accepting that maybe her perception of love was a lie, and that maybe there was another Pinkie out there who loved them even more than she did).
Twilight's distraction test did seem pretty random and merciless, but the real Pinkie
was able to clear it by using the love for her friends as motivation to stay focused, and was able to shut out anything and everything around her and concentrate 100% on the painfully boring task of watching paint dry, knowing what would happen if she failed. From her perspective, losing meant being separated from her friends. For the copies, it just meant they'd have to go back and no longer be able to pursue their artificial desire for fun. Whatever the copy Pinkies were, hopefully on the other side of that lake they'll be free from that artificially-imposed desire.
BTW, there's no indication that the real Pinkie "barely" passed the test. If Twilight hadn't ended the test, the real Pinkie might've continued staring at that wall well into the next morning, long after even her judges passed out from exhaustion. If they did that, people would've been less inclined to doubt her, but there wasn't really a need.