"The seventh car is not in front of the garage, but in the garage."
Translation:A hetedik autó nem a garázs előtt áll, hanem a garázsban.
why dont I need a second elott one before and one after a garazs ?
előtt doesn't come before anything; it comes after things. It's a postposition.
So you can have two if there are two things for it to come after, e.g. az előtt a garázs előtt "in front of that garage".
But there is no az "that" in Duo's sentence, so there's no additional thing for an előtt to come after.