1) if many are found that look exactly alike -- the doubling might actually be in the raised lettering on the device that creates the lettered edge.
2) If the examples are all a little different, then the cause might simply be the same as other double edge lettering dollars - except having only a very light spread (i.e. two times through the lettered edge machine without any significant rotation of the orientation of the letters).
If #1 is true, then this error might be a closer 'kin' to a doubled die or die deterioration doubling then it is to double edge lettering.