A sender MUST NOT generate multiple header fields with the same field name in a message unless either the entire field value for that header field is defined as a comma-separated list [i.e., #(values)] or the header field is a well-known exception (as noted below).
Specifically, the content-length header should be disallowed. Headers like Cache-Control are allowed.
Per 3.2.2
Specifically, the content-length header should be disallowed. Headers like Cache-Control are allowed.