Wouldn't this be pronounced as du-uu? So a short "du" because the د has this ُ above it, and then a long "uu" (or "wuu") because the first و is so to say prolonged by the second و. Right? The way she says it here I feel like it would have to be دُوُ?
The idea is that in Arabic the vowels e and o do not exist. In Arabic there are only three vowels: a, i, u (however, sometimes "a" is pronounced like "e"). So, words like Duo cannot be written in Arabic. That's why there is a double و: the first one is the u (dUo) and the second one is the closest vowel to the sound "o" (duO).