Triple Strike
The image to the left is an example of a
Lincoln Cent that has been triple struck.

In the normal process of striking coins, a
blank planchet is struck once by the dies,
then ejected.  If a coin is not ejected properly
it can be struck again.  Coins can also be
struck multiple times.  Furthermore, multiple
strikes can also happen both inside and
outside the collar.  

Image courtesy of Heritage Auction Galleries