To Rainbow Dash, it seemed like it was going to be like any other day. A few minutes of pushing clouds around (or 10 seconds, if she really exerted herself), and then some hanging around Ponyville with nothing to do, except impress her friends with some tricks, of course. Maybe some minor crisis would happen, too, and she’d learn something about the value of friendship, or something. All of that changed when she’d finished clearing the sky for the day and saw Twilight bouncing around with glee in a very un-Twilight-ish fashion. Rainbow zoomed down to see what was going on.

“Uh, hey Twilight! How’s it going?” she greeted, trying to mask her confusion.
“Great! I’ve never felt better! There’s a really cool new book I wanna show you,” Twilight giddily replied, “You wanna see it?”

Rainbow responded: “Uhh, ok? What book is it?”

"It’s the new, uhhh, Daring Do book! Yeah! It has loads of adventure and fun!” Twilight suspiciously declared, stifling a laugh.

“Ok, I’ll try it. Maybe when I get back home I can get reading.” Rainbow offered.

“But don’t you wanna read it nooow?” Twilight pleaded.

“Uhh, sure. Not sure why this is so important, though.” Rainbow replied tentatively, as she picked up the book. As she attempted to open it, the cover refused to come off the pages. She yanked harder at the cover, until finally, she could see that the cover was stuck to the pages with bubblegum, as were the pages themselves. She finally got a look at the cover, which read in obviously handwritten text in pink crayon: “Daring Do and the Best Prank Ever! By me, Pinkie Pie!!!”

Rainbow’s eyes nearly popped out of their sockets. “Uhh, Twilight, what’s going on?” Twilight snorted, then burst out laughing. “Silly Dashie! I’m not Twilight!” she said, as suddenly, she began to shift. Her horn and wings disappeared, her hair poofed up, her color changed to bright pink, and her cutie mark shifted from stars to three balloons. “I’m PINKIE!!!”

Rainbow’s jaw dropped as she exclaimed: “WHAT?? How’d you do that?? What happened? Where’s Twilight? Is she dead? What did you do to her?”

“Twilight’s fine, silly billy! It was just a prank! When I woke up this morning, I figured out I could look like anypony I wanted!” Pinkie reassured.

“Wait, I thought only changelings could do that! Have you secretly been a changeling this whole time or something??” Rainbow fretted.

“No, silly! I guess I’m just that cool!” Pinkie responded.

Rainbow, after realizing everything was ok, and telling herself this wasn’t any weirder than anything else Pinkie’s done, she calmed down a bit. “Yeah, I guess that’s pretty cool, Pinkie. Wait, so you can look like anypony you want, right?”

“Yes-siree-bob!” Pinkie affirmed.

“I’ll bet you can’t do me. I’m just too awesome to-” Rainbow started, before turning around and looking directly into the eyes of an exact replica of herself. “Woah! You look just like me! But less cool.”

“I know! This is so fun! ‘I’m Rainbow Dash! I’m so awesome, I can count to infinity. Twice.’”

“That is so not how I talk.” Rainbow defended.

“Is too!”

“Is not!”

"Is too!”

“Is not!”

“Is too!” the two jabbed at each other, as the citizens of Ponyville attempted to ignore the sound of two Rainbow Dashes arguing in the street.

After a few minutes of back-and-forth with her double, Rainbow conceded: “Fine! You win this one, Pinkie. But can you do Applejack?”

“Yup!” Pinkie giggled, as she quickly changed into the splitting image of Applejack, minus the hat. “Howdy-do-there, y’all! Ah’m Applejack, jus’ about the most truth-tellin’ pony around! Yee-haw!”

“Nice! How about Rarity?”

“Yeah! ‘Oh, dahhhling, you simply must see my new collection! It’s simply the finest ensemble mode in all of Equestria!’” Pinkie cooed, as she took on the white unicorn’s visage.

“Ha ha! Now do Fluttershy!” “Ok!” Pinkie called, changing into the yellow pegasus. “Wait. I can’t make fun of Fluttershy! She’s way too nice to be made fun of!” she confessed.

“Oh, yeah. Right. Well, you know what else we could do?” Rainbow replied.

“What?!” Pinkie asked excitedly, changing back to her pink self.

“More pranks!”

Meanwhile, Rarity was hard at work in her boutique. She’d promised to make her friend Twilight a new dress for her Hearts-And-Hooves Day celebration, and she wanted it to be absolutely perfect. She’d asked her to come over to see how it was turning out and make any requests she had. As the door opened and she walked in, Rarity turned to face her friend and greeted: “Why, hello, Twilight dear! I assume you’re here about the dress I’ve been working on for you? I want it to be simply the finest I can make. Nothing less for a friend!”

“Thanks, Rarity! But don’t get too carried away there. You don’t need to worry. I’m sure whatever you can do will be great!”

“Why darling, I insist! Great simply isn’t good enough for you, dear. Something a bit more... fantastic... fabulous... MAGNIFICENT!” Rarity exclaimed.

Twilight giggled a bit at Rarity’s dramatic display. “Alright, Rarity. Well, I’m glad to know you care so much about me and my celebration. And I’ve got to say,” she said, motioning to the unfinished dress, “it’s looking amazing already!”

“Why, thank you, dear! But what I really need is your input. It’s no good having the best dress in all of Equestria if it isn’t really yours, of course!” Rarity replied.

“Well, it’s really hard to think of how to improve it when it’s turning out so great already! Maybe a bit of ribbon there, and a star pattern there, and it would be perfect!” Twilight suggested.

“You’re absolutely right, darling! Of course, there’s simply no point in just thinking about it! Wait outside a bit while I get started, I’ll call you back in when I’m finished. And if you have any other suggestions or questions, just ask me!” Rarity beamed.

After only a few minutes of working on the dress, however, Rarity heard a knock at the door again. “Come in, dear!” she called. As she trotted over to the door, she saw Twilight enter her shop once again. “What is it, Twilight?”

“Hi Rarity! Whatcha working on?” Twilight bubbled, in a somewhat out-of-place manner for her.

“Why, I’m still working on your dress, dear! The one for Hearts-and-Hooves Day? Remember?” Rarity replied, trying her best to hide her bewilderment.

“Oh yeah! I totally remember! Anyway, I was just looking for my baritone, and I was wondering if I’d left it here.” Twilight remarked, seemingly trying to stifle a laugh.

“Why, darling, I didn’t think you a musical type! Of course, dear. I haven’t seen anything like that here, but I will let you know if I see it.” Rarity responded, still confused.

“Thanks! Byyee!” Twilight called, as she zipped out of the door.

About 5 minutes later, Twilight once again entered the boutique. “I know you told me to wait, but I was too excited to see how it was turning out!” Twilight admitted, as she trotted toward her friend.

“Oh, hello again, Twilight! Have you had any luck finding that baritone of yours? I still haven’t seen it here, but you’re welcome to take another look!” Rarity greeted, trying to make sense of the situation.

“Baritone? I don’t own a baritone!” Twilight remarked, baffled.

“You don’t? Then why in Equestria would you ask if I’d seen one but 5 minutes ago?” Rarity countered, just as completely in the dark as the princess.

“I didn’t ask if you’d seen a baritone! Are you sure that was me?” Twilight insisted.

“Of course darling, how many other purple alicorns live in Ponyville?” Rarity pointed out.

“Alright, Rarity, but I didn’t ask about a baritone. Maybe you should take a break if you’re starting to misremember things.”

“But darling-” Rarity started, but Twilight’s hoof came to her mouth to stop her.

“A break, Rarity. Please?” Twilight insisted.

“Alright, but I’ll get back to your dress as soon as I feel better!” Rarity called after her, as she began to leave the shop.

This time, only about a minute passed before Twilight re-entered the shop.

“Hiya, Rarity! Didya find my baritone yet?” she asked, giddily.

“Wait one minute, I thought you told me you didn’t own one?” Rarity asserted, half confused and half frustrated.

“No I didn’t, silly! Anyway, let me know if you find it!” Twilight giggled, as she walked out the door once again.

The next time Twilight entered the shop, Rarity had simply had enough. “NO! I do not know where your baritone is, whether or not you own one!” she bellowed.

“But I thought I sa-” Twilight started, before being cut off by an enraged Rarity:

“I know you said you didn’t own one! But you have come in here twice asking if I had seen one! What is going on? What is happening? I DEMAND AN EXPLANATION!!” she shouted, before promptly having her own questions answered by a strange sight: an exact double of Twilight trotting into her boutique.

“Hiya Rar-” she started, before noticing Rarity and the first Twilight gaping at her. “Oh. Oopsie.” she muttered.

“Who are you?”

“Why have you been impersonating Twilight?”

“And most importantly… HOW?!” After a barrage of questions, Pinkie finally decided to reveal herself.

“Alright, I’ll tell you. Hey Rainbow! You can come in now!” In walked Rainbow Dash, laughing her wings off.

“Hahaha! You should have seen the look on your faces! I wish I had a camera! That was priceless!” she snorted. “But darling, what on Earth is go-” Rarity began, before being interrupted by the second Twilight shifting suddenly into the shape of Pinkie Pie.

“Yep! It’s me! I really gotcha good, didn’t I!” Rarity was befuddled, but Twilight’s face seemed to indicate a broken brain.

“Are you alright, darling?”

“Yeah, are you feeling ok?”

“Come on, silly, it was just a prank!” Twilight sat in silence for a few seconds, before suddenly beginning a panic attack. “B-b-b-b-b-ut… a...a...changeling…in Ponyville???... impossible...how...why...must... warn...the...PRINCESS!!!” Twilight shouted, as she ran out of the boutique at speeds most pegasi racers could only dream of. Just another day in Ponyville.

Author's Note

Just a shorty to get me started. Sorry if this one feels a bit pedestrian. This is kind of just to get me started here. Also, over-the-top formatting is kind of my thing when writing comedic stuff, so sorry if it annoys you. I'd love feedback on this! Honestly, if literally anyone sees this and says anything about it, then it's served its purpose.
Thanks for giving this a try!