Correct spelling, explanation: similar is the correct spelling as it derives from French similaire, also spelled with one m. Double m form, simmilar is probably just a typo and is incorrect. The correct form is similar.

Definition of similar:
adjective, alike
Their cars are very similar and I was confused for a second.
These contracts were of a similar nature so there wasn’t much work with them.

Incorrect spelling, explanation: spelling similar with double m (simmilar) is an incorrect form. This adjective has come to English from French similaire hence the correct spelling: similar. There is no pronunciation nor lexical reason to spell this word with two m’s.

Incorrect spelling

Incorrect spelling

Incorrect spelling

Incorrect spelling

Incorrect spelling

Incorrect spelling