Combine two lists while removing duplicates

The goal is to combine (append) two lists while removing the duplicates.

let rec find_dup a lst =  match lst with    | [] -> false    | hd::tl -> if (hd == a) then true else find_dup a tl;;let rec app lst2 lst1 =  match lst1 with    | [] -> lst2    | hd::tl -> if (find_dup hd lst2) then (app tl lst2)     else hd::app tl lst2     ;;

I have my code like this but when the test case isapp [4;5;6;7] [1;2;3;4] the answer should be [1;2;3;4;5;6;7]but I keep getting

 - : int list = [1; 2; 5; 3; 6; 4; 7]

What is going on?