I took a look on the Internet myself just out of curiosity. The case of Emilie Sagee and her double is quite famous and is repeated in numerous books on the supernatural. Ms Sagee is said to have been a teacher at a fee paying school near Wolmar called “Pensionnat von Neuwelcke” from 1845 and among the many schoolchildren who supposedly saw her doppelganger two are named, Julie von Guldenstubbe and Antoine von Wrangel.
The original source for all these stories seems to be Robert Dale Owen’s Footfalls on the Boundary of Another World
It is interesting that Robert Owen travelled to Paris shortly before publication of his book to consult Baron de Guldenstubbe, who was himself the author of a recent treatise on spirit writing and who appears to have been Julie von Guldenstubbe’s brother. All that we know of Ms Sagee is what she related to Robert Dale Owen on this visit.
Julie Wilhelmine von Güldenstubbe (1827–1888) lived with her brother in Paris. She worked as a medium and was editor and publisher of his scientific writings.
I could find no trace of the school where the incidents supposedly took place.
Emilie Sagee was supposedly born in Dijon in the Burgundy region of France in 1813 and there may be a birth record for her. That would be proof of her existence.