According to About.com, a double-headed coin is a novelty item. These types of coins are created by hollowing out the center of a coin, leaving the ridged edges and obverse side undamaged, and filing down a second coin to fit tightly into the shell.
About.com notes that the United States Mint does not print double-headed coins. The U.S. Mint has safeguards in place to make sure that coins are created properly. The die shafts used to press coins are sized so that they can only fit into the coin press in a certain way, such as a "heads" die shaft that is round and a "tails" side shaft that is square. Because the differently shaped or sized shafts will not fit into the opposite holes in the press, double-headed or double-tailed coins cannot be made by the U.S. Mint.