"This is perfect! Everyone will now bring their fun to me."
"Ah, Pinkie... you have got to stop talking to yourself. Starting... now."
"Into her reflection she stared,
yearning for one whose reflection she shared,
and solemnly sweared
not to be scared
at the prospect of being doubly mared."
The very first time they showed her duplicating herself was nicely done, and creepy -- she chanted the rhyme, dove into the pool, and emerged into the mirror world, where the real Pinkie Pie was waiting.
"Goodness! Was it something I said?" "YES!"
There was a nice callback to the Smile song with ponies poking out the windows to smile and wave... until they saw the pinkie swarm and retreated inside.
"If I can't tell us apart, who can?"
Answer: Anypony with a brain. The duplicates didn't have any of Pinkie Pie's memories and only a ridiculous charicature of her personality, but none of her friends *or Pinkie Pie herself* realized that the only Pinkie with any depth was the real thing.
"Please, the real Pinkie Pie never sat in one place that long in her whole life."
Clearly, the writer was making fun of fanfics (or episodes) where all Pinkie Pie did was bounce around causing trouble and chanting 'fun fun fun!'
...actually, if you want fridge horror, the very first mirror-Pinkie Pie was more upset and worried than the real Pinkie Pie before they made any extra duplicates, so it's possible they sent the wrong one back. c.c But the real Pinkie Pie *was* the only one who showed any consideration at all for her friends, and they had a clear line of continuity so that the viewer never actually lost track of her, so probably not.
The thing I liked least about the episode was the stupid way Twilight and Spike found the spell to send the mirror-Pinkies back. It was even stupider than the seed of truth thing, but at least they moved on past it quickly to the actually-not-that-awful scene where they had to figure out which pinkie was real.
Overall... well, it was a pretty standard 'army of clones' episode, but the relatively subtle 'everyone underestimates Pinkie Pie' theme saved it I think.