This can't be happening. Spike? A girl? And not even a changeling? It's just impossible! Twilight could do some research on this strange phenomenon, but she is too shocked to even think straight. Thus, her only course of action is to at least let her friends know what is going on, so they may be able to help her, or at least keep it a secret so as not to cause a panic amongst the townsfolk. Pinkie Pie may be her nearest living friend, but given her unpredictable nature, Rarity's place, the second nearest of her friends' homes, is the safest bet for a start.
Speaking of the townsfolk, as Twilight rushes to Carousel Boutique, she notices an unusual sight: several stallions she passed by look just like mares that live around town, but she thinks nothing of it. She's on a mission, after all.
As soon as she arrives at the boutique store, she frantically knocks on the door, and before she calls out, it's opened by a stallion. But this stallion was far abnormal, because he looks disturbingly just like Rarity: same white coat, similiar violet, wavy mane and tail, similiar elegant, blue eyes, even the same cutie mark on his hips!
"Why, if it isn't Twilight! What seems to trouble you, my dear?"
Twilight sinks further into shock, and stammers, "W-W-What the hay?! Who are you, and where's Rarity?!"
"Rarity? Who's this 'Rarity' you-"
Wasting no time to even pin this imposter down, Twilight blasts him with another anti-changeling spell, knocking him onto the table behind him, scattering everything on it with a loud clatter, but just like with "Spines", there was no other effect.
This guy shakes himself back to his senses, then steadies himself back on his hooves before exclaiming, "My goodness, mare! What possessed you to such an assault?"
"So...", says Twilight, dumbfounded, "...you're not a changeling, either?"
"Of course not! Wait, 'either'? What do you-"
"Does this mean you really are Rarity?"
The stallion takes a moment to formulate a response, considering Twilight's emotional state, then delivers: "Twilight, darling, it's me. Elusive. Don't you remember? Since we first met while I was decorating City Hall for the Summer Solstice Festival?"
"But, back then, you were-"
"What the hay?!" A little colt cries out from the footsteps of the stairs leading up to Rariy -er- Elusive's private dwelling. He must have heard the noise and noticed the scattered contents of the table. Also, this boy looks alarmingly like Rarity's little sister, Sweetie Belle: same white coat, similar curly, pink-and-violet mane and tail, and the same blank flank. "What just happened here?"
"Silver Belle, mind your tongue!" Elusive snaps at the colt, takes a breath, and calmly replies, "...and go back upstairs. Twilight and I are having a serious discussion."
"Actually, I, er, I think I should go check on the others. Take care!" And without another word, Twilight galloped out of the door to who knows where.
"Twilight, wait!" But she is already gone. Elusive sighs, and looks back at the table. "Perhaps I should get this mess cleaned up?"