The requested Chat participant is already mapped to a conversation.
Possible Causes
You are attempting to add a Programmable Chat participant to a conversation that already has another participant with the same identity.
Possible Solutions
Retrieve a list of existing participants in this conversation and remove the one with the same identity using REST API.
Perhaps it's not an issue, and we can continue as-is. Instead of adding a new participant, reuse the existing one, potentially update its attributes if necessary.