50416: Participant and proxy address pair is already in use

The requested participant and proxy address pair is already registered in the system.

• You are attempting to map a non-Chat (e.g. SMS) conversation member with participant address and proxy address that are already assigned to an active conversation. For example, there may be only one unique mapping between your conversation participant's public phone number and internal (proxying) Twilio phone number, in order to route inbound messages correctly.

• Locate a conflicting active conversation that has an identical participant added to it. Either delete old conversations completely if you don't need the message history, or just remove the participant that uses these numbers in a previous conversation.