dumb and stupid have different forms for their superlatives*:

dumb
dumber
dumbest

stupid
more stupid
most stupid

Never use stupider or stupidest.

*The form of an adjective that expresses which of more than two items has the highest degree of the quality expressed by the adjective; in English, formed by appending “-est” to the end of the adjective (for some short adjectives only) or putting “most” before it. — Wiktionary

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