Symptoms:
VxAccessTokenAlreadyUsed (20120) error received when using a Vivox Access Token (VAT).
Resolution:
Always Use a New VAT
A new VAT must always be generated when performing a VAT-required action (such as signing in, joining a channel, or performing an admin mute). You must generate new VATs for any new operations. Even if a user is signing in again with the same exact username, or is joining the same exact channel name that they were previously in, you must generate a new VAT.