The Most Normal of Days in Ponyville
The Crusaders wanting a special secret pool hang out for themselves, discover the "Top Secret Passage to The Mirror Pool" sign with easy access path, equipped with railings, behind a bush to the very boulder Pinkie laid into the hole she fell through.
Having found this new place, they vowed to make it their new club hideout. After a few failed attempts at cannon-balling as they'd instantly re-emerge and bust their rump onto the stone flooring around the pool, the fillies soon discover the place might not be the best place for a pool party. All of which agreed the other set of Cutie Mark Crusaders, followed by further nods of the next few sets of Cutie Mark Crusaders behind those Crusaders.
Fast forward past the shock and awe and into the awesomeness of what one can do with extra copies of themselves, the fillies seek to use this to try more ways to get their cutie marks much in the way any butt mark seeking pony would do: Try their hooves at ALL the talents!
Meanwhile, unbeknownst to the Crusaders... two sneaky shadows followed by one nosy dragon, they soon discover that Ponyville might well be facing its greatest threat on a whole new scale. Legions of Cutie Mark Crusaders find themselves bored and restless, seeking any and all new things to try and experience, many forgetting their original masters' reasons for doing so, are now running rampant across all of Ponyville. The Mane 6, especially AJ, Rarity, Rainbow Dash, and Twilight Sparkle are besides themselves on how to wrangle them up.
As Twilight attempts to banish a few random crusaders back to the pool she is heavily persuaded to rethink her course of actions by three overprotective mares.
Meanwhile, at the Pool Cave, Spike is yet again being a Spike. Ignoring this, the trio are met by Discord who points a simple solution about how to save Equestria from the Cutie Mark Crusaders, because apparently Fluttershy says he isn't allowed to banish the herd to some random dimension, something about being inequinal or some such pony sentiments, and blurts out the crusaders one weakness: Diamond Tiara and Silver Spoon. Stating the two of them are the only beings that can restore balance to the cosmos: Diamond Tiara and Silver Spoon.
Spike being the Spike he is, chooses to ignore Discord's blunt and detailed truths, all without revealing too much, and decides a gentler and kinder approach would work best. So Spike pulled a Spike thing and now there was a venerable stock of extra Spikes.
Some bucks to the face and general embarrassment as all the Spikes end in obvious Spike fashion, he finds himself at the mercy of the The Legion of Marks-less Flanks, ready to meet his fate when a sudden cackle and guffaws blast over the drowning "YAY"'s of crusader clones. All the clones becoming stock still, as if frozen in time. Each one turning gulping a few milliseconds apart causing a rather chilling effect as the air was rife with the scent of dread. A level of fear not seen since that day Spike had to climb halfway down Twilight's throat to save her from choking to death on a really, really-really, cheesy casedia.
As one, the Legions of the Mark-less Flanks all turned towards the grating and mocking laughter, the small act of all those ponies turning at the same time would later cause a few devastating hurricanes and tidal waves to destroy quite a few small islands that would have given material for a certain Daring adventurer to use in her books.
What they saw next was one of their own, an Apple Bloom of unassigned ranking, sniffling and dainty kicking the ground beneath her with a hoof. The want and need to continue crusading for her mark in a "Demolition Cutie Mark" all but gone. At this, the legions broke ranks and followed their strongest instincts: Fear
Waves of Silver Spoons being commanded by two General Commanders and their Queen. One of the Diamond Tiaras commanded the Spikes that were just moments ago being made fools of, the other two commanding the waves of Silver Spoons, each flanking and corralling all the crusaders that have dispersed and to the center of town hall.
With insults and hard truths at their disposal, the Grey and Purple armies did little work of the Legions of the Mark-less Flanks.
With the day saved... by Diamond Tiaras and Silver Spoons and some Spikes. Alls well that ends well.
Or so it seemed. Turns out even after finding the originals, the many crusaders and Silver Spoons and the Spike clones, are unaffected by Twilight's dispelling magics. "I don't understand, why isn't this working?"
"I'm afraid that's due to an Equinal Law I passed back during olden times I'm afraid."
"Princess Celestia!" said Twilight Sparkle. The waves of gasps and squeals from the Diamond Tiaras and Silver Spoons nearly deafened the crowd. Regaining her composure, Twilight continued. "So, what does that have to do with these clones? And my magic?"
"Long story short Twilight, there were some desperate ponies who used to close many things. Some even cloning their children in order to see which would help them further their own wants and gains. After time, it came to my attention that the clones were often times seen as defective or disposable at a family's whim. They would be zapped and trapped with their all the memories of their experiences growing up their other selves. Never knowing why or how they came to be."
"That's... wow. Way to put a downer on the whole clone thing, Princess." Stated Diamond Tiara. None having seen which of the three had spoken.
"So, yes, it was quite the 'downer' at the time. Some families even using the spell to rid themselves of the original who didn't meet up to their family's expectations. So I intervened and altered the very enchantments themselves in order to instill a protection within the very pool itself. It would resist the wants and dispel all magics that would attempt to force a child born of the mirror pool back again if the construct had no desire to return of its own free will.
With Twilight unable to send the clones back, not with Celestia's new law that passed into place for young magical constructs. "But if we can't just zap all these fillies and Spikes back into the mirror pool due to some Equinal Law on zapping underage magical constructs, how will we ever get Ponyville back to normal_er?"
And for that one moment in time, the planets aligned. The last word in the book of tales was penned just as its author took his last breath, one colt finally found the lost battery needed to play his gamecolt. The ponies at attention all came to one single conclusion.
"DIS~CORD!!!"
And so, thanks to Discord, the fillies and Spikes of the Mirror Pool were given their own dimensional home to frolic, and do Spike and filly things. Fluttershy and Discord working hard as rain to find homes and things for the fillies and spikes to do.
At least until they all turned 18 in their respective species equivalent age and would be deemed legal enough for Twilight Sparkle to zap that.