I prefer Alternative B, which is the text Russ has written.
ALTERNATIVE B: + <t>If the proto-article already had an Injection-Date header + field, it MUST NOT be modified or replaced. If the + proto-article had both a Message-ID header field and a Date + header field, an Injection-Date header field MUST NOT be + added, since the proto-article may have been multiply injected + by a posting agent that predates this standard. Otherwise, + the injecting agent MUST add an Injection-Date header field + containing the current date and time.</t>