Audio device dupe warning

As reported in [this forum thread](https://forum.libcinder.org/topic/cinder-audio2-crash-device-not-reachable#23286000002088119), some odd machines (this one presumably being a hacintosh) seem to report the same UID + name, so there is no known way to create a unique key on the cinder end. So, this PR reduces the check to a warning and continues on happily.

This isn't the end of the world - getting the default device should still work, `findDeviceByName()` and `findDeviceByKey()` would both return the first Device that it found that matched the query string. Guess its the best we can do for now.