Correct spelling, explanation: the word fulfill was already known in Old English as fullfyllan, corresponding to full and fill. However, in Middle English, the form was changed and it was written with one consonant l in the element ful as fulfillen. And nowadays, we have the form fulfill and only this is correct.

Correct spelling, explanation: this spelling is preferred mostly outside North America, correct also in Canada.

Definition of fulfill:
verb, when you fulfill something, you bring it to realization; it means also that you satisfy requirements or obligations and bring to an end,
Peter hasn’t yet fulfilled the requirements in order to get the job.
If she were responsible, she wouldn’t fail to fulfill her obligations.

Phrases with fulfill:
fulfill one’s promise – accomplish something; perform what you said that you would do,
I’m so tired but I have to fulfill my promise.
He is unreliable. He never fulfills his promise.
fulfill oneself – achieve your aspirations or ambitions,
David is trying to fulfill himself as a musician.

Incorrect spelling, explanation: there is the word full meaning containing a lot and fill to make something complete. Some learners of the language can join these words to make fullfill but such form does not exist in English. Actually, the word consists of a verb fill and the forming element ful. In Modern English, the combinations with ful are more common at the end of the word. But the words fulfill and fulsome are the only survivors where ful is at the beginning. This is the reason why fullfill is incorrect. It should be spelled as fulfill.

Incorrect spelling

Incorrect spelling