Flow should recognize and complain about duplicated case statements

[SheetJSDev]

This is currently accepted but really should be an error (at least, closure compiler recognizes the error):

function foo(x: ?number): string {
  switch(x) {
    case 2: return "foo";
    case 3: return "bar";
    case 3: return "baz";
  }
  return "default string";
}

[nmn]

To the best of my knowledge, this currently a low-priority thing.