Book 13 of the Seishun Buta Yarou series.

IS THE REAL MINISKIRT SANTA IN A SEA OF FAKES?

Sakuta thought he’d already accomplished step one: Find Touko Kirishima. The next step was convincing her to help protect Mai, but that proved easier said than done. In the process, however, he makes a confusing discovery: There are multiple Touko Kirishimas! The Touko he knows may not actually be the person he’s looking for! With clues from his friends’ prophetic dreams, he gets closer to figuring out exactly what danger awaits Mai, but he’ll have to sort through the fake Kirishimas and determine who’s the real deal before it’s too late…

(Source: Yen Press)