REST HTTP status codes for failed validation or invalid duplicate

Question

I am building an application with a REST-based API and have come to the point where I am specifying status codes for each requests.

What status code should i send for

Answer 1

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.