One of the most oft-asked questions rare coin dealer hear is regarding double stamped coins otherwise properly know as a double die coin. Specifically it means that the doubling is in the die itself. This is a serious error on the part of the Mint staff. When this happens many coins are struck with the same defect. The more dramatic the error, the most it tends to be worth.
Checkout the classic and iconic 1955 Double Die Lincoln Cent on this post.
See our page on how to Verify the Authenticity and Grade of your rare coins.