Correct spelling, explanation: fulfill is the American way to spell the British word fulfil. The American form may be caused by the existence of the word fill, spelled with double l and having the close meaning to fulfill. In other English-speaking countries, especially in UK and Australia, the word is spelled fulfil (in Canada both forms are used), however, there is no such form as fullfil.

Correct spelling, explanation: it’s spelled this way outside North America, correct also in Canada.

Definition of fulfill:
verb, to meet one’s expectations/needs, etc.; to come to an end, usually in a positive way
I finished work for today and I can finally fulfill my today’s need of watching the game with the beer in my hand.
Jerry fulfilled my dream and bought me a Birkin bag for our anniversary!

Incorrect spelling, explanation: this mistaken form probably comes from the differences in spelling of the word fulfil (or fulfill) in the English-speaking world. There are two ways of spelling: American fulfill and British fulfil – these both forms are correct, just used usually in different parts of the world. Fullfil however doesn’t exist and is simply a misspelling.

Incorrect spelling

Incorrect spelling