let rec is_ordered l = match l with
  | a::b::q -> a <= b && is_ordered (b::q)
  | _ -> true;;
let rec is_rev_ordered l = match l with
  | a::b::q -> a >= b && is_ordered (b::q)
  | _ -> true;;

let rec has_ordering l = match l with
  | a::b::q -> if a < b then
      is_ordered (b::q)
    else if a > b then
      is_rev_ordered (b::q)
    else has_ordering (b::q)
  | _ -> true;;

assert (has_ordering [1; 1; 2; 2; 3]);;
assert (has_ordering [3; 2; 1; 1]);;
assert (not (has_ordering [1; 1; 2; 1]));;