Parallelism With Prepositions

When using a preposition in a list of items, the preposition only needs to be used once.

Catherine is making a chocolate cake with whipped cream and with strawberries.

If we were thinking of this sentence as a set of scales (think of the old-fashioned ones with two little dishes hanging from a bar), Catherine is making a chocolate cake would be on one side, and whipped cream and strawberries would be on the other. With would be the bar across the top. If we keep that second with, the little dishes are no longer balanced.

Catherine is making a chocolate cake with whipped cream and strawberries.

I have a long to-do list: I need to clean the house, to do the errands, and to pay the bills.

Even if the preposition to is being used to form the infinitive, there’s no need to repeat it.

I have a long to-do list: I need to clean the house, do the errands, and pay the bills.

For weeks after the Annual Neighborhood Marble Tournament, we found marbles under the chairs, under the tables, and in the corners of the room.

Because a second preposition (in) is required, it’s alright to repeat the second under; however, it could be left out: …we found marbles under the chairs and tables, and in the corners of the room.