Ocaml - parameter type when checking for duplicates in a list

I've got a basic function which checks a list for duplicates and returns true if they are found, false otherwise.

    # let rec check_dup l = match l with
        [] -> false
      | (h::t) ->
        let x = (List.filter h t) in
        if (x == []) then
           check_dup t
        else
           true
    ;;

Yet when I try to use this code I get the error

    Characters 92-93:
          let x = (List.filter h t) in
                                 ^
    Error: This expression has type ('a -> bool) list
           but an expression was expected of type 'a list

I don't really understand why this is happening, where is the a->bool list type coming from?