dup killer = D = dusty deck
dup loop /d[y]oop loop/ (also `dupe loop') n.
[FidoNet] An infinite stream of duplicated, near-identical messages on a FidoNet echo, the only
difference being unique or mangled identification information applied by a faulty or incorrectly configured system or
network gateway, thus rendering dup killers ineffective. If such a duplicate message eventually reaches a
system through which it has already passed (with the original identification information), all systems passed on the
way back to that system are said to be involved in a dup loop.
--The Jargon File version 4.3.1, ed. ESR, this entry manually entered by rootbeer277.