The 1911-D/D $20 is one of the more interesting varieties in the Saint Gaudens series. It doesn't rise to the level of a Major variety, but it is distinctive enough to attract the attention of collectors, and there are ample supplies to meet that demand. A die-sinker at the Philadelphia Mint (remember, the "mother" mint at Philadelphia made all the dies for the branch mints) created this variety when he punched the mintmark into the die twice, with the positioning of the second punch slightly off from the first.
This variety can be found nice; Gems and even MS66's are common. The best example is a single PCGS MS67+ now held by Bob Simpson. No other example above MS66 is known.
This is not an arm's-length variety, so be sure to examine any 1911-D Double Eagles for a repunched mintmark. It's worth the effort, as this variety is worth approximately 150-200% of a regular type coin.