From BlenderWiki

Duplicate

This is the most simple tool, as it just duplicates the selected elements, without creating any links with the rest of the mesh (unlike extrude, for example). Once the duplication is done, only the new duplicated elements are selected, and you are automatically placed in grab/move mode, so you can translate your copy elsewhere…

Note that duplicated elements belong to the same vertex groups as the “original” ones. The same goes for the material indices, the edge’s Sharp and Seam flags, and probably for the other vertex/edge/face properties…