More than one person already noted this, but I will add a bit of clarification.
What is an RPM? The mint used to create all of the working dies from a working hub that did not have any mint mark on it. Then, before the die was to be used, a mint employee would use a punch with the mint mark on it to apply the mint mark to the die. Sometimes, the mint mark was punched in the wrong place or possibly not well struck and the mint worker would have to punch it again. The second punch may not be in the exact same place as the first one. It is also possible that the punch would bounce when it was hit and left more than one impression on the die.
This explains how an RPM is created. The two impressions on the die come out as 2 mint marks on the coin. That is also why the placement of the mint mark can vary greatly from coin to coin.
About 1990 the mint started putting the mint mark on the hubs and no longer punching them on the individual dies. Therefore, there can no longer be an RPM.
Here is a picture of a mint worker applying a mint mark to a die.
