Learn Polish: Kiedy byłam u lekarza, taksówka odjechała od budynku. - When I was at the doctor's, the taxi drove away from the building.

Odjechała specifically means drove away or departed. The prefix od- adds the meaning of moving away from a starting point. If you used pojechała, it would just mean the taxi went or left, without emphasizing the separation from the building. It is perfective because it describes a single completed action.

Is it normal to repeat od in odjechała od?

Yes, this is a very common pattern in Polish! When a motion verb has a direction prefix like od- (away from), it is frequently followed by the matching preposition, in this case od (from). This combination highlights the separation.

Why is budynek in the form budynku here?

The preposition od requires the Genitive case. For inanimate masculine nouns like budynek (building), the Genitive ending is usually -u. Also, notice how the -e- near the end drops out so budynek becomes budynku, which is a common vowel drop in Polish nouns.

Why does the sentence use u lekarza instead of a preposition like w or na?