You have cited an extraneous preposition, i.e., one that could be removed from a sentence without any loss or change of meaning. Generally speaking, it's best to omit them altogether. Here are a few more examples:
We need to clean (up) the kitchen.
He's standing inside (of) the fence.
I saw him jump off (of) the bed.
Sometimes, superfluous prepositions are found at the end of sentences or questions; these should be omitted:
Where are they (at)?
Where are they going (to)?
For the non-native speaker, though, such prepositions can be hard to detect, because some are necessary, or idiomatically correct. For example, in the following sentences, the prepositions in parentheses should not be removed:
He agreed (to) meet me at the cafe.
The song of the robin differs (from) the song of the sparrow.
They will show (up) sometime after lunch.
Suddenly, it dawned (on) him: "I forgot my keys!"