Diffs two lists preserving duplicates

ListDiffWithDups.fsx

module List

let rec diffDups minuend subtrahend =

  let rec removeFirst i = function
    | h::t -> if h = i then t else h::(removeFirst i t)
    | [] -> []

  match subtrahend with
  | h::t -> diffDups (removeFirst h minuend) t
  | [] -> minuend


#time
let test =
  [ diffDups [] [1;2;3] = []
    diffDups [1;2;3;2;4] [] = [1;2;3;2;4] 
    diffDups [1;2;3;2;4] [2;3] = [1;2;4]
    diffDups ["a";"b"] ["a";"b"] = []
    diffDups [5;6] [6;5] = []
    diffDups [2;3] [1;2;3;2;4]  = []
    diffDups [1;2;1;2;4] [1;1] = [2;2;4] 
    diffDups ([0..10]@[10..100]) [0..10] = [10..100]
    diffDups [1..2..300000] [1..2..299990] = [299991..2..300000]
   ]
   |> List.forall id