How did Blackbeard have 2 fruits?

Blackbeard somehow copied the Gura Gura no Mi powers, similar to how Dr. Vegapunk copied (to a certain extend) the Pika Pika no Mi from Kizaru and gave it to the Pacifista. In essence, this isn’t actually having two Devil Fruit powers, but instead a copy of the earthquake power.

Is Blackbeard the only person with 2 Devil Fruits?

Teach, most commonly referred to by his epithet Blackbeard, is the captain-turned-admiral of the Blackbeard Pirates, currently one of the Four Emperors. He is also the only known person in history to wield the powers of two Devil Fruits.