This is a noobish question but how do I separate duplicated meshes? When I duplicate a mesh and select one of them, all of them get selected automatically. Thanks.
if you are duplicating inside edit mode then it is still part of the one object
you can duplicate in object mode to create a new object or in edit mode - duplicate - then with the verts you want as a new object press P
also if you duplicate with shift d - it will make a copy
alt d - will make a “linked copy” - so if you edit one all will change
it works thanx 
knigknog,
If you would only like part of a mesh, you can select the verts, shift d to duplicate, then p to separate the selected verts to a different object. Also, if you have joined a couple of objects, in edit mode, select a single vert, then press ctrl l, to select all of the linked verts. Then press p to separate.
Best of Luck!