Idempotent Methods - PUT, DELETE, and safe request methods

Idempotent Methods and understanding their behavior is critical to getting REST right and nothing explains it better than reading the spec about what it means.

4.2.2. Idempotent Methods

   A request method is considered "idempotent" if the intended effect on
   the server of multiple identical requests with that method is the
   same as the effect for a single such request.  Of the request methods
   defined by this specification, PUT, DELETE, and safe request methods
   are idempotent.

http://tools.ietf.org/html/rfc7231#section-4.2.2