Hasn't a fork happened before?

Yes. In July, bitcoin underwent a similar fork that led to the creation of bitcoin cash.

There was an initial surge in price, and it hit an all-time high of $914.45, but has fallen steadily, according to data from Coinmarketcap.com. On Wednesday, bitcoin cash was trading just above $330.

Bitcoin cash's market capitalization — the total value of the cryptocurrency in circulation — is just over $5.5 billion, compared to more than $93 billion for bitcoin.