For unknown reasons, GVC mixer control can sometimes signal a new
device with the same id as one that was added before. This means that
in device_added_cb, a duplicate entry with that id is created, in my
case with a different name. However, the last one added is valid, but
that one cannot be selected because all other logic in the sound panel
assumes the first hit in get_iter is valid. This breaks sound input
selection then.
The fix is easy; only add a new list entry if none with that id exists.
Fixes #2403 (closed).