Conflict

Returned when a write operation would violate a unique constraint — a duplicate slug, primary-key collision, or any database-level uniqueness rule. The detail field identifies the conflicting value.

Example RFC 7807 body

{
  "type":   "https://ferra.rs/errors/conflict",
  "title":  "Conflict",
  "status": 409,
  "detail": "title 'Casablanca' is already in use"
}