Incorrect spelling, explanation: becomming is an incorrect spelling because we do not double m in become when we use suffix -ing. Instead, it is spelled becoming. Since the root verb ends with e, m makes no sense to double, because it is not the last letter. E is skipped because according to the rule we omit it with suffix -ing. Taking all of these into consideration, becoming is the correct form.

Correct spelling, explanation: becoming is the correct form because it comes from the root verb become, which ends with the vowel e. In the case of e ending, when suffix -ing is added the vowel is omitted. M does not double, because the base word is not becom (it would double in such case). Therefore becomming is incorrect and the correct form is becoming.

Definition of becoming:
adjective, suitable, proper; also: about clothes or haircut
This jacket is very becoming on Mary.
It’s not becoming to talk like this to your grandparents.
noun, change, or process of change
Acting in a play means becoming another person.

Incorrect spelling