Relevant to Blender v2.31
To duplicate an object, press SHIFT-D to create an identical copy of the selected objects. The copy is created at the same position, in Grab mode.
This is a new object except that it shares any Material, Texture, and IPO with the original. These attributes are linked to both copies and changing the material of one object also changes the material of the other. (You can make separate materials for each, as described in the Materials Chapter.)
You can create a Linked Duplicate rather than a real duplicate by pressing ALT-D. This will create a new object with all of its data linked to the original object. If you modify one of the linked objects in EditMode, all linked copies are also modified.