Twilight looked at Princess Twilight, then at Sunset Shimmer, then at Spike. She then looked at her own hooves. "This...this is..." She shook her head. "This is impossible..."
"No, this is magic," Sunset said. "And you'd best open your mind and accept it, because..." She trailed off, looking around. "Uhh...Twilight? Princess Twilight, I mean...where the hay are we?"
"Oh! This is the library in my castle, just outside of Ponyville." She smacked herself in the side of the head with a hoof. "That's right! I forgot to tell you I had the mirror moved here."
Twilight looked at the mirror which stood behind Princess Twilight. The center had a rippling, glowing vortex of energy. Various devices that looked like something a mad scientist might cobble together stood arrayed around the mirror, glowing and puffing and whirring and sparking. "What's all that stuff?" she asked.
"Oh, that?" Princess Twilight asked. "That's what's keeping the portal open. On its own, the portal only opens every thirty moons. The last time it opened was..." She tilted her head, doing some mental math. "About eight moons ago. It opens for a few days, then closes for another thirty moons."
"Wait a minute," Twilight said, suddenly alarmed. "This portal only opens every two and a half years?! So...so if I'm here too long, or..." She paused. "I could get stuck here?!"
"No, no!" Princess Twilight laughed. "Don't worry about that! That's how the portal used to work. I found a way to open it at will." She gestured to the bizarre construct surrounding the mirror. "I designed this apparatus myself. It draws dimensional conduit magic from a secondary source and channels it into the mirror. The secondary magical feed bypasses the temporal limiter, allowing the dimensional conduit to activate at any time, for any duration." She pointed at the book set prominently at the top of the device. "That journal provides the secondary magical feed!"
Twilight blinked slowly, processing this. "You...you're using a book as a battery?"
"Yes!" Princess Twilight said happily, grinning.
Twilight stared from the portal apparatus, to Princess Twilight, an expression of wonder dawning on her face. "You are me," she breathed.
Spike started laughing. Sunset rolled her eyes.
Twilight tried to walk, but her legs shook. "I...I'm really a horse right now..."
"Pony," the others corrected.
"Unicorn, specifically," Sunset said. She frowned. "Hey, what's up with that? Shouldn't she be an exact duplicate of you, Princess?"
Princess Twilight shook her head. "This Twilight hasn't had the experiences I've had. She hasn't made the friends I've made, or discovered the magic of friendship...the magic that made me a princess."
Shakily, Twilight walked up to Princess Twilight...and craned her neck to look up at her. "You're...tall," she said.
Princess Twilight blushed. "I've hit kind of a growth spurt since becoming an alicorn," she said. "Moreso in recent weeks."
"Alicorn?" Twilight asked.
Princess Twilight spread her wings. "Alicorns have the magic of all three pony races, plus their own unique, powerful magic. Oh, and we're immortal." She made a face. "Not really a fan of that last one."
"What's so bad about being immortal?" Sunset asked.
Princess Twilight gave her a sad look. "I can give you five reasons," she said.
"Five...?" Sunset's ears drooped. "Oh."
Princess Twilight's horn glowed, and a reflective magical surface appeared in midair. "Go ahead, Twilight. Take a look at yourself."
Twilight took a few shaky steps, trotting over to the temporary mirror and inspecting herself. "That...is weird," she said.
"You're taking this a lot better than this Twilight did when she went to your world the first time," Spike said. "She screamed her head off!"
Twilight shrugged. "Well...all of you told me this would happen," she said. "Just because I didn't believe it at the time doesn't mean I wasn't listening. I'm freaked out and have a million questions, yes, but...screaming about being turned into a unicorn, however impossible that is, won't accomplish anything."
Princess Twilight wrapped a wing around her. "We'll try to answer all your questions. I promise."
Sunset looked down at herself. "Hey...I've still got my jacket!"
"Oh, I forgot!" Princess Twilight said. "I've been making some minor tweaks to the portal's material transfer protocols." She pulled a face. "The first time I went through that thing I lost my best set of saddlebags."
"Ouch." Sunset's horn lit up, and her phone and wallet floated out of her pockets. "Oh, good, I've even still got this."
Princess Twilight nodded, grinning. "Yep! Although I don't think you'll get reception here."
Sunset laughed. "Probably not. Still, at least I can take some pictures."
The flapping of wings sounded from the corridor. The doors burst open. "Hey Twilight, got a minute to fix Tank's harness? It's drifting off to the left..."
The two Twilights and Sunset turned to the blue pegasus who had just breezed into the library. She blinked. "Uhh...Twilight? Why are there two of you?" she asked slowly.
"Oh, hey Rainbow Dash," Princess Twilight said. "This is the Twilight Sparkle from the other world. And you remember Sunset Shimmer, right?"
"Kinda," Rainbow said. She looked at Sunset. "Uhh...'sup?"
"Hey Rainbow Dash," Sunset said awkwardly. "Uhh...good to meet you?"
Rainbow landed. "I thought there wasn't another Twilight over there?" she asked.
Princess Twilight sighed. "There is. I just didn't know because, well...she doesn't go to CHS."
"Which is good for us, because that woulda made things a lot tougher both times," Spike said. There was a sudden flash of emerald green.
Princess Twilight frowned. "Spike? What did you just do?"
"Oh, nothing," Spike said, putting his claws behind his head and looking innocent.
"Uh-huh." Princess Twilight looked at Rainbow. "Can it wait a little while? I've got a little...situation here."
"Sure, no big," Rainbow said. "I'll drop him off with Fluttershy until you can fix the harness."
"I promise I'll get to it as soon as I can. Probably tomorrow." Princess Twilight stroked her chin thoughtfully. "Hey, Rainbow Dash? When you're done dropping Tank off, would you mind coming back here? You could show Sunset Shimmer around Ponyville." She looked at Sunset. "How's that sound?"
Sunset nodded. "Sure. I've never been to Ponyville. Besides, it'll be nice to stretch my hooves, get outside, smell Equestrian air and grass."
"Cool. I'll be back in a few," Rainbow said, flying off again.
"That was...Rainbow Dash?" Twilight asked slowly. "The Rainbow Dash in our world is the captain of several sports teams and plays guitar in the Rainbooms..."
"And the Rainbow Dash here is a pegasus, part of Ponyville's weather team, and a reserve Wonderbolt," Princess Twilight said.
"Weather team?" Twilight asked. "She's a meteorologist?"
Sunset laughed. "Not exactly," she said. "In Equestria, pegasi control the weather."
Twilight's eyes widened. "Weather control? Are you serious?"
"Except for a very few places in our world, all nature is controlled by pony magic in Equestria," Princess Twilight said. "I'll explain it all to you while Sunset Shimmer is out exploring Ponyville."
"A-alright..."
A bright golden flash filled the library. "I came as quickly as I could," a new, mature, musical voice said. "What's the emergency?"
Twilight stared. A majestic white alicorn stood in the middle of the library, angelic wings spread wide. Her horn was exceptionally long, and her pastel hair flowed in a nonexistent breeze.
The blood drained from Sunset Shimmer's face. "Princess Celestia," she whispered thickly.
Princess Twilight gave Spike a measured look, raising an eyebrow. Spike tilted his head at Sunset and grinned sheepishly.
Celestia looked around the room, eyes widening at the sight of two Twilights...then she saw Sunset, and she gasped. "S-Sunset Shimmer?!"
Sunset ducked her head, pawing the crystal floor nervously. "Uhh...so..."
Celestia was upon Sunset in an instant, sweeping her up into a hug. Tears spilled from her ancient, expressive eyes. "I never...I always hoped...oh, thank goodness..."
"Princess...I...I'm so sorry!" Sunset wailed, tears streaming from her eyes as she sunk into Celestia's embrace. "Everything I said and did...the way I acted...I..."
"Shh," Celestia said, stroking the back of Sunset's head with a hoof. "It's alright, Sunset Shimmer. All is forgiven. I...I'm just relieved to see you again."
Princess Twilight sidled over to Spike. "Nice work," she whispered.
Spike smiled. "I thought it was time."
After a long moment, Celestia drew back, examining Sunset. She laughed. "What...what is this you're wearing, my little pony?"
Sunset looked down at her jacket. "What, this? Oh...I've had this ever since I went through the mirror the first time. Twilight's tweaked the portal so stuff like this can pass through safely..." She looked up at Celestia. "You...you're really forgiving me? Just like that?"
Celestia sniffled. "Just like that," she said. "So...are you...are you back to stay?"
Sunset sighed. "No," she said. "I don't belong here anymore. I...my home is in that other world now. My friends are in that other world."
Celestia sighed. "I...I understand," she said. "Still...I had hoped..." She wiped a tear from her eye. "You...you will at least come back to visit more often, I hope?"
Sunset smiled. "Of course."
"Good," Celestia said. "I've..." She swallowed. "Every day since you left, I've...I've regretted driving you away. You're the only daughter I've ever had..."
"DAUGHTER?!" Princess Twilight exclaimed.
Celestia looked over at her. "I...never told you, because that part of the story was...was too painful," she said. "Not long after I met Sunset Shimmer...I adopted her as my daughter. That's...that's why her cutie mark is..." She blinked as the presence of a fourth pony in the room registered. She stared at the unicorn Twilight with glasses who was shifting awkwardly in the corner. She frowned. "Twilight? Is...is there something you'd like to explain?"
Princess Twilight nodded. "Princess Celestia, meet the Twilight Sparkle from the other world," she said.
"I had wondered," Celestia said. "Greetings, Twilight Sparkle. I am Princess Celestia, ruler of Equestria. I...I expect you have a great many questions..."
"Yes I do, Your Highness," Twilight said. "This other Twilight's going to be answering them."
"Of course," Celestia said. She frowned. "Forgive me, Princess Twilight, but...why bring her here?"
"She started investigating Canterlot High School and Princess Twilight," Sunset said. "She ambushed me in the school bathroom. I did the only thing I could think of. I called Princess Twilight for help."
"I see," Celestia said. "I suppose this has all been very confusing for you," she said to Twilight.
"To say the least," Twilight said. "Especially once magic, other dimensions, and talking ponies got involved."
Celestia laughed.
Sunset butted her head against Celestia's chest. "Mom?" she asked. "I'm...I'm going to take a walk around Ponyville with Rainbow Dash while Princess Twilight talks to her double. Do you...would you like...?"
Celestia smiled. "Of course. Shall we wait for her outside, then?"
Sunset nodded, smiling. "A little help with the teleport? I can't aim..."
"Of course," Celestia said, giggling. In a bright flash, the two disappeared.
Twilight stared, eyes wide. "Did she say...teleport?"
Princess Twilight nodded. "It's not a common ability," she said. "I've only known a few unicorns other than myself who could even do it...well, and the Princesses, of course." She sat down. "So...shall we get started?"