What are the comparative and superlative forms of the adjective "stupid" ? Some native speakers say "more stupid" and "most stupid". Is it correct ?

Both forms are correct. In fact, comparatives and superlatives with more and most are gaining ground with two-syllable adjectives :

commoner/more common; stupid/more stupid;politer/more polite.

Languages DO change ! 

'The Oxford English Dictionary of current English' gives stupider and stupidest as the comparative and superlative forms because the adjective has only two syllables. 

Yes, it's fully correct because we can't add "er" or "est" to the end of this word.

I think "more stupid" and "most stupid" are correct forms.