What this coin looks like (obverse, reverse, mint mark location, special features, etc.): The 1956 D Lincoln Wheat Cent (Penny) actually has a variety of repunched mintmark (RPM) errors of all sorts. One of the more major errors is the D above a "shadow" D mintmark. The best way to identify this error is to notice a very subtle but clear "D" located fully under the primary "D" mintmark. This is caused by the die striking the coin twice but the "D" from the first strike still appears on the coin after the second strike. This "shadow" D is pointed out between the arrows in the example image of this coin above.
Coin value chart: Typical coin prices, values and worth in USD based on grade/condition