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REST HTTP status codes for failed validation or invalid duplicate

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

What status code should i send for requests failing validation or where a request is trying to add a duplicate in my database?

I've looked through http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html but none of them seems right.

Is there a common practice when sending status codes?

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When issuing an HTTP DELETE request, the request URI should completely identify the resource to delete. However, is it allowable to add extra meta-data as part of the entity body of the request?

HTTP vs HTTPS performance

Are there any major differences in performance between http and https? I seem to recall reading that HTTPS can be a fifth as fast as HTTP. Is this valid with the current generation webservers/browsers? If so, are there any whitepapers to support it?

What's the difference between Cache-Control: max-age=0 and no-cache?

The header Cache-Control: max-age=0 implies that the content is considered stale (and must be re-fetched) immediately, which is in effect the same thing as Cache-Control: no-cache.

Google has failed to solve this mystery for me :(

What's the difference between a POST and a PUT HTTP REQUEST?

They both seem to be sending data to the server inside the body, so what makes them different?

Is it valid to replace http:// with // in a