I'll guess one of your subscribers, or their mail system, is bouncing 
your message back to the list.  You may need the help of your Listserv 
site administrator to see if this is the case and find the subscription 
causing the problem.

cheers, wayne

John Mack wrote, in part,  on 9/26/2006 12:11 PM:
> Every time I submit a message to PHARMA-MKTING listserv (I am the 
> moderator and owner), I get the following error message:
>
> Your message  is being returned to  you unprocessed because it  
> appears to have
> already been  distributed to the  PHARMA-MKTING list.  That is, a  
> message with
> identical text  (but possibly with different  mail headers) has been  
> posted to
> the list  recently, either by  you or  by someone else.  If you have  
> reason to
> resend this message to the list (for instance because you have been 
> notified of
> a hardware failure with loss of data),  please alter the text of the 
> message in
> some way and resend  it to the list. Note that altering  the 
> "Subject:" line or
> adding blank lines at  the top or bottom of the message  is not 
> sufficient; you
> should  instead add  a  sentence or  two  at  the top  explaining  why 
> you  are
> resending the  message, so that the  other subscribers understand why  
> they are
> getting two copies of the same message.
>
> My message gets posted to the list OK. I do not send duplicate 
> messages as far as I am aware. I also get this error message when I 
> send from my hotmail account (which is a regular subscriber account). 
> This leads me to believe that everyone who posts a message gets the 
> same error, which can be confusing.
>
> Any ideas how I can stop this?