Clarify duplicate properties in JSON Batch payload

How should duplicate properties (i.e., id, method, url, headers, dependsOn, atomicityGroup, body, if) in a JSON Batch payload be handled?

Most of these (id, method, url, atomicityGroup, body) should pretty clearly be errors as they are single-valued.  However, RFC2616 allows the same header to appear multiple times in a request only if the header value is a coma-separated list of values, and states that the multiple headers is equivalent to a single header with the values appended, in order, separated by comas. So, a service could allow the "header" property to appear multiple times and append the values. We could do the same thing with dependsOn, and even if.

Or, we could keep it simple and say that a client must not specify any of these properties more than once.