Source : Free On-Line Dictionary of Computing

dup loop
     
        /d[y]oop loop/ (also "dupe loop") [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 killer}s
        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}.
     
        [{Jargon File}]