Use these options to set what happens when you select Edit Mesh > Duplicate. 
- Separate duplicated faces 
 
- Separates faces automatically after they are duplicated. The duplicated faces are separated into their own separate object. Any pre-existing shells in the mesh will not be affected. 
 
- Offset 
 
- Enter a value to offset the edges of the duplicated faces. Offsets for extracting and extruding faces are also shown here for comparison. This option can be used to produce a bevel effect for extrusions, cut-outs around faces using Extract, and to uniformly scale duplicated faces.