409 Conflict
Meaning
The request could not be completed due to a conflict with the current state of the target resource.
When to Use
When a request cannot be processed because of a conflict, such as an edit conflict in the case of multiple updates or a violation of resource state assumptions.
Example
Attempting to create a resource that already exists (and uniqueness is required) can result in 409 Conflict, indicating the conflict between the new resource and an existing one.
When Not to Use
Do not use for general validation errors (422 may be more appropriate if it's semantically correct data that can't be processed) or for conflicts that are best represented by a specific 4xx code (like 412 for precondition failures).
Source
RFC 9110