REST HTTP status codes for failed validation or invalid duplicate

by

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

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