Make adding an existing MultiProgress member a no-op - #854

The docs on MultiProgress::add and the insert* methods say that adding a bar that is already a member has no effect (#503), but internalize() always allocates a new slot, so the bar moves and its old slot is never freed. If that slot is first in the ordering, finished bars are never pruned and a dropped bar keeps being redrawn (two spinners with a {msg} template):

let pb1 = mp.add(spinner("pb1").with_finish(ProgressFinish::AndLeave));
let pb1 = mp.add(pb1); // documented as having no effect
let pb2 = mp.add(spinner("pb2"));
pb1.tick(); pb2.tick();
pb1.finish_with_message("done"); drop(pb1);
mp.clear().unwrap(); pb2.tick();
// screen before: "done\npb2", after: "pb2"

This returns early from internalize() when the bar already belongs to this MultiProgress, using the same Arc::ptr_eq check as remove(). So insert(0, pb) on an existing member no longer moves it, which matches the docs.

The new mp_double_add_is_noop test fails on main (5 members instead of 2). Tests pass with all, default and no default features; fmt, clippy and doc are clean.