Yes, DUAL is a Scrabble word!

Letter values in Scrabble US: D 2, U 1, A 1, L 1.

Is it a valid word?

DUAL is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

consisting of or involving two parts or components usually in pairs
having more than one decidedly dissimilar aspects or qualities; public preaching and private influence"- R.W.Emerson
a grammatical number category referring to two items or units as opposed to one item (singular) or more than two items (plural)

Also: two-fold, three-fold.