if an inpout is repeated in a list delete the first occurenece code example

Example: Remove duplicates from a list (keep first occurrence)

list_rset([], []).                     % keep rightmost occurrences
list_rset([E|Es], Rs0) :-
   if_(memberd_t(E, Es),
       Rs0 = Rs,
       Rs0 = [E|Rs]),
   list_rset(Es, Rs).

list_lset([], []).                     % keep leftmost occurrences
list_lset([E|Es], Ls) :-
   post_pre_lset(Es, [E], Ls).         % uses internal auxilary predicate

post_pre_lset([], _, []).            
post_pre_lset([E|Es], Pre, Ls0) :-     % 2nd arg: look-behind accumulator
   if_(memberd_t(E, Pre),
       Ls0 = Ls,
       Ls0 = [E|Ls]),
   post_pre_lset(Es, [E|Pre], Ls).