This was a Dream Sequence

@Cabrony  
Well… since pony Twilight left the human world after the events of the third film, I guess Princess Twilight’s fall formal dress now belongs to the human Twilight. I guess even the Wondercolt attire, Rainbooms outfits, Slumber party pajama’s (which were belonged to Human Pinkie Pie I guess), and the Rockin Hairstyle uniform also now belong to Human Twilight too. I don’t know why Twilight’s bringing more than one dress to Camp Everfree. And I mean I so some supporting and background characters in their fall formal outfits in the movie’s finale.

Where did Pony Twilight’s cloths come from?
 
Where did the dress go after Pony Twilight returned to Equestria?
 
When Pony Twilight returned to CHS Why was she wearing her default cloths, that she left at Rarity’s salon, and not what she should have been wearing last, the dress? Then again rarity said “don’t forget the dress” in Friendship Game so it’s a animation error / lack of continuity.
 
Pony Twilight should have been in the Dress when she returned to CHS. She should have given it back to Rarity, and retrieved her default outfit. It ether an oversight, same dress, reference. The fact EG Rarity’s entire character revolves around fashion; it is conceivable that Rarity could have made more than one.
 
Oh, this all happened in a Dream… There is a psychic subconscious connection between Ponies and the Human counterparts, usually manifesting in the Humans being drawn to Cuttie Mark like symbols.

@Cabrony  

 
My thoughts exactly!

Slowpoke
 
Also it’s a Dream
 
Also they just reused assets

@Cabrony  
Rarity could have made a double and given it to her.

so why does sci-twi have pony twilight’s dress in her room?