I'm too dang tired to quote and trim Rik's post, but it seems to me that
if two of the recipient headers are identical, then on the recursion test
after finding the second occurrence, procmail will start its match after the
first appearance again and get into a loop, continuing to think it is finding
more recipients until (since Rik intentionally removed the bailout-when-
enough-is-enough code) it runs out of file descriptors and aborts.
That's why it was worthwhile to store the rest of the head, from each match
on, in a variable instead of retesting on the entire head. It also reduces
the search area.