What this coin looks like (obverse, reverse, mint mark location, special features, etc.): There are a quite a few varieties of the 1960 Lincoln cent penny and this is one of the more major and valuable ones. The US Mint stamped the small date on top of the large date. The best way to identify this error is to note the small zero within a fainter larger zero. Sometimes you can notice a small doubling of the 6 as well. There is also a "ghostly" D mintmark stamped just above the solid D mint mark (D/D). All of these are depicted by the arrows in the example image above.
Coin value chart: Typical coin prices, values and worth in USD based on grade/condition