sort: allow a sort mode to appear multiple times

A test case for this is `sort -k 2n,2n` which should be accepted, because `n` is
compatible with itself.

Resolves issue #4129.

tests/by-util/test_sort.rs

@@ -1156,3 +1156,8 @@ fn test_tmp_files_deleted_on_sigint() {
     // `sort` should have deleted the temporary directory again.
     assert!(read_dir(at.plus("tmp_dir")).unwrap().next().is_none());
 }
+
+#[test]
+fn test_same_sort_mode_twice() {
+    new_ucmd!().args(&["-k", "2n,2n", "empty.txt"]).succeeds();
+}