message: Replace some cloned() with duped()
  Summary: Makes it clear where the values we are duplicating are cheap to do so.
diff: @@ -183,10 +183,10 @@ mod tests {
  #[test]
  fn few_entries() {
  let entries1 = vec![(0), (1)];
  - let m1 = entries1.iter().cloned().collect::<SmallSet<_>>();
  + let m1 = entries1.iter().duped().collect::<SmallSet<_>>();
  let entries2 = vec![(1), (0)];
  - let m2 = entries2.iter().cloned().collect::<SmallSet<_>>();
  + let m2 = entries2.iter().duped().collect::<SmallSet<_>>();
  assert_eq!(m1.is_empty(), false);
  assert_eq!(m1.len(), 2);
  assert_eq!(m2.is_empty(), false);
  