When a blank planchet is struck by the dies, the normal procedure is for the feeders to eject the struck coin out of the collar and into a chute. If there is a malfunction and the struck coin isn't ejected, it may receive a second or third strike by the dies. A multiple struck coin can happen in many ways and have many combinations of errors.
Major mint errors on U.S. gold coins are extremely rare. There are only a few known that are double or triple struck. One phenomenal example is this double stuck 1904 $20 rotated 180 degrees in the collar. Considerable detail remains from the initial strike and is visible under the portrait on the obverse and the eagle on the reverse. This exciting mint error was in my personal collection and has been authenticated and certified by NGC and BYERS COLLECTION is noted on the insert.