A recurring header that satisfies this clause of the Spec:
Multiple message-header fields with the same field-name MAY be present in a message if and only if the entire
field-value for that header field is defined as a comma-separated list [i.e., #(values)]. It MUST be possible
to combine the multiple header fields into one "field-name: field-value" pair, without changing the semantics
of the message, by appending each subsequent field-value to the first, each separated by a comma.
A recurring header that satisfies this clause of the Spec:
Multiple message-header fields with the same field-name MAY be present in a message if and only if the entire field-value for that header field is defined as a comma-separated list [i.e., #(values)]. It MUST be possible to combine the multiple header fields into one "field-name: field-value" pair, without changing the semantics of the message, by appending each subsequent field-value to the first, each separated by a comma.