Vowels in Hebrew words follow certain patterns. The choice of a particular patten largely depends on the part of speech and the grammatical form of the word given. By listening how the words are pronounced in sentences you will gradually get used to those patterns and will learn to read withoud the nikkud, thus bypassing the primary school stage.
It's actually doubled specifically to tell you it's pronounced as a "v" sound instead of an "U" sound. Basically if you see a double Vav in the middle of a word it's going to pronounced "v". A bit weird, I know but still makes way more sense than a lot of English spelling and pronunciation . ;)