webhook_target_exists
A target with that webhook_id is already registered. The id is the spoken name the model calls it by, so two targets may not answer to one; PATCH the existing target or choose another id.
Problem shape
Every non-2xx response is an RFC 9457 application/problem+json body. The type URI below is stable and machine-matchable, so branch on the final path segment (the code) and treat the status as redundant confirmation. See the error-handling guide.
{
"type": "https://docs.vocapable.com/errors/webhook_target_exists",
"title": "…",
"status": 409,
"detail": "…",
"instance": "/v1/…"
}