audioio.AudioOut constructor accepts the same pin for both left & right channels

[jepler]

.. but I think it should give an error instead, possibly at the shared-bindings level.

This came up in review of #9995, which includes a special case. When fixing this, remove that code when you do.

[dhalbert]

If it would sum/mix (without overflow) the two channels, then that's useful functionality. If not, it should be an error.