DUALHDR is one of a set of mutually exclusive header options, so choose
whichever one of the others (FULLhdr,FULLBsmtp,IETFhdr,SHORThdr,SHORTBsmtp)
you want.  See line 47ff of the current LISTSERV REFCARD.
 
Mark R. Williamson, Rice U., Houston TX; [log in to unmask] or @RICEVM1