Cause
If-None-Match was sent for an external_id that already exists with a higher source_revision.
What to do
Update with If-Match and the current etag.
application/problem+json
{
"type": "https://masnet.com/developers/errors/already_exists",
"title": "Already exists",
"status": 409,
"detail": "...",
"code": "already_exists",
"correlation_id": "..."
}