Duplicating a mesh lets you quickly create a set of similar objects, create a collision object closely based on the original shape, or take a snapshot of your mesh at any point in time as you continue sculpting. Duplicating then flipping a mesh lets you easily create symmetrical pairs of objects (like gloves, boots, and so on).
To duplicate a model
This duplicates all selected meshes. If no mesh is selected, it duplicates the active mesh.
Mudbox creates a copy of the model(s) in the same location, and automatically selects the copy. Because the new mesh initially overlaps the original exactly, you cannot initially see the copy.