with import <nixpkgs/lib>; 

let

  isFirst = list: x: end:
    if end == 0 then true else
    if elemAt list (end - 1) == x then false 
    else isFirst list x (end - 1);

  uniq' = list: concatMap (i: let
    value = elemAt list i;
  in optional (isFirst list value i) value) (range 0 (length list - 1));

in
  uniq' [ 3 3 4 5 3 4 1 ]