So I'm making a hallway, and I'm going to the model several places in my map, but I don't want to texture every single one of them. I tried to unwrap a part of it, but how do I copy that unwrap and use it again?

You're going to tile the hallway geometry (that's what I got from that)?
I'm sure there's a way, but it'd be so much easier if you just textured/mapped it before tiling it.

when you have the geometry unwrapped how you like.
in the unwrap "edit" window, go to the top tabs, usually , file,view, blah.
i think its in the 1st or 2nd one.
copy
then select the other geometry (providing it is an exact copy)
paste (from the same tab, in "edit" unwrap window)
or unwrap one piece, then clone and move it.

Not sure how much you have done in this program, but also, when unwrapping a model, it is good to collapse the UVW map to the editable mesh/poly when you are done.
This will make it so that your UVs are locked in and don't reset if you decide you need to edit 1 small piece in the mesh later on.

to collapse, right click where it says UVW unwrap, and you will see the menu button for "collapse to" the UVW map tab will go away, but you can always open it again if you need to re-unwrap the edited area.