REST HTTP status codes for failed validation or invalid duplicate
For input validation failure: 400 Bad Request + your optional description. This is suggested in the book "RESTful Web Services". For double submit: 409 Conflict
Update June 2014
The relevant specification used to be RFC2616, which gave the use of 400 (Bad Request) rather narrowly as
The request could not be understood by the server due to malformed syntax
So it might have been argued that it was inappropriate for semantic errors. But not any more; since June 2014 the relevant standard RFC 7231, which supersedes the previous RFC2616, gives the use of 400 (Bad Request) more broadly as
the server cannot or will not process the request due to something that is perceived to be a client error
- Failed validation: 403 Forbidden ("The server understood the request, but is refusing to fulfill it"). Contrary to popular opinion, RFC2616 doesn't say "403 is only intended for failed authentication", but "403: I know what you want, but I won't do that". That condition may or may not be due to authentication.
- Trying to add a duplicate: 409 Conflict ("The request could not be completed due to a conflict with the current state of the resource.")
You should definitely give a more detailed explanation in the response headers and/or body (e.g. with a custom header - X-Status-Reason: Validation failed).
I recommend status code 422, "Unprocessable Entity".
11.2. 422 Unprocessable Entity
The 422 (Unprocessable Entity) status code means the server understands the content type of the request entity (hence a 415(Unsupported Media Type) status code is inappropriate), and the syntax of the request entity is correct (thus a 400 (Bad Request) status code is inappropriate) but was unable to process the contained instructions. For example, this error condition may occur if an XML request body contains well-formed (i.e., syntactically correct), but semantically erroneous, XML instructions.