Hi! You get all the Advantages listed when you choose a background. If Sorcery is listed more than once, you get however many additional levels of Sorcery are included in the Background. (For example, if Sorcery is listed twice, you get whatever additional benefit selecting Sorcery two times provides).
I don't have the page number, but I think this is clarified in the Sorcery section of the rules rather than the Hero creation section. Hope this helps!
I think page 150 is just saying "While some other advantages may be taken only once, you may take Sorcery multiple times" because don't forget that you can select a single level of Sorcery from your 5 points of Advantages in addition to any Sorcery you get from a Background.
If a player really wanted two particular non-sorcery Backgrounds as well as magic, I could certainly see them taking one level of Sorcery as part of their 5 points of additional advantages. For example, a Pirate/Sea Captain who also wanted a bit of Glamour Magic. Just as you don't have to take the Duelist Background to get a sword school, you can spend all 5 points of your customized Advantages and buy a school separately after choosing other Backgrounds. I would guess it's set up that way to give players extra flexibility in creating characters with Sorcery.