https://tools.ietf.org/html/rfc7230#section-3.2.2 says:

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).

This ties sender behaviour to how the header field is specified; do we require headers to be defined using ABNF?

If so, we should say so explicitly. If not, we should clarify this requirement.

Furthermore, Section 7 ties handling of specific syntax (e.g., empty lines) to the #-rule. If headers can be defined without ABNF, we should clarify whether that behaviour is required, or specified to #-specified headers.

See alsohttpwg/http-extensions#596