Use the Duplicate option when you want to create a discontinuity in the mesh in a particular place by duplicating nodes. This is interesting, for example, when dealing with very thin shapes where it is difficult to represent the domain with two overlapped surfaces, and it is easier to have a single surface marked with this meshing option (like sails embedded in a volume, material cracks, etc.). 
Marked lines and the deformed mesh after an a structural analysis
It is possible to mark lines embedded in a 2D domain, or surfaces in the case of 3D domains.