A simple ML algorithm for removing duplicates in a list is given below :

remdup.sml

fun remove_duplicates (xs) =
	case xs of
	    []=>[]
	  | x::xs' => if List.exists (fn y=> x=y) xs'
		      then remove_duplicates (xs')
		      else x::remove_duplicates (xs')

val test_remove_duplicates = remove_duplicates ([3,3,3,3,5,5,5,5,6,6,6,6,7])