Yes, duo is a Scrabble word!
...and is worth 4 points in Scrabble, and 5 points in Words with Friends
duo
Noun
1. Two items of the same kind
2. Two performers or singers who perform together
3. A pair who associate with one another
4. A musical composition for two performers
What Scrabble words can I make with the letters in duo?
3 Letter Words
2 Letter Words