Number of messages a member of multiple lists will receive

Hello,
  If you have 3 lists,
1) list1
2) list2
3) list3

If  [email protected] is a member of list1 and list2 and list3 and in the 
list1 there is a statement to include list2 and list3  members
(i.e :include:/usr/local/mail/lists/list2
:include:/usr/local/mail/lists/list3 ).

I beleive [email protected] will receive only one message sent
to list1 by another list1 member. If this is correct which program 
handles this so that no duplicates will be sent to the same member 
belonging to different lists. Is it majordomo (if so which particular 
majordomo program) or sendmail (I'm using). Can someone please provide 
some pointers/links/explanation of the process taken.

Thanks
Allan