I copied a bit from an article in The Chronicle issue of November 1971.
Quote:
the "false" doubles are relatively common on the plate
while the true doubles are uncommon an d very striking varieties. In order to
qualify as a true double, both lines must run from the top to the bottom of
the stamp design (or nearly so) , both mu st be of equal weight, and they must
not touch each other. True doubles are the result of recutting of both lines that
make up the double.
For clarity I would add, the "plate" mentioned is Plate 15.