How do I make an object always be a duplicate of anything? I can duplicate objects, but I need one to be an exact copy of the mesh at all times. That is, whenever object 1 moves because of an animation, gravity, logic, etc, the second will do the same. Other than just making a full duplicate so they have the same logic, how can I do this?
If you duplicate the object in question using ALT+D as oppose to Shift+D The objects will be linked. Anything done to either object should be reflected with the other(s).

Edit:
Do note wiki.blender.org is a viable resource when looking for information like this.
The entry found here directly pertains to your question.