Original text:
. A recipient MUST ignore the If-Unmodified-Since header field if the <del> received field value is not a valid HTTP-date, or if the field value has more than one member. </del>  A recipient MUST interpret an If-Unmodified-Since field value's timestamp in terms of the origin server's clock.

Reviewer comments:
evaluating false on a duplicate change request sending 2xx without validator for reasons unknown.
Hmm, of course, now I found another related message in my archives that prompted me to remember why the requirement was added. I should probably rewrite the paragraph (without that requirement) and describe the edge case as a note.
RFC7232 in URL has the following clause: It doesn't really explain what security or performance considerations are leading to such a requirement, but it calls for a lot of infrastructure and design work to implement it because that logic can be implemented only if the server is stateful. The stateless server cannot comply with such a requirement. See related mail list thread: URL Either RFC should explain why such logic MUST be implemented or relax "MUST" into "SHOULD" in that clause or remove that clause completely.
Should I submit an errata proposing to remove the following clause from RFC 7232, section 3.1: " In the latter case, the origin server MUST NOT send a validator header field in the response unless it can verify that the request is a duplicate of an immediately prior change made by the same user agent" It seems like nobody can recall/explain why this clause is there.
I finally found the original commit 303b82b12a53c170f6f755aaaf8840ac1240aa77
And the discussion motivating the change is under . Certainly the intent was to remove the constraint that an error be sent when a duplicate request is made, so the extra limitations were to reduce edge cases.
It is not an errata. This is the right place to discuss it and either find a better wording or strike the text.
Thanks, NAME for finding the original commit and your response! Sending response for repeating requests makes total sense. My issue is about a sub-condition of response that is stated in the RFC7232. In the original commit that you referenced that sub-condition is stated as: I think I understand the original recommendation not to send back. It would be better to replace with there though. But in RFC7232 that sub-condition clause is changed to: The reason of not sending with back is not explained in RFC7232 and nobody can remember why it is there or explain this requirement. Complying with this requires dev effort and extra infra since we would need to track immediate previous requests from each agent. Our API servers are light-weight and stateless so this is a huge requirement for us. Instead, we would prefer a way simpler approach of always sending back along with responses, but we need to comply with RFC7232 thus this discussion. So, the proposal is to do one of the following (preferred) Remove the following clause from RFC7232: (If aforementioned clause is kept) Explain the reason of not sending in that sub-condition (at least) Change to in that clause.
These requirements were motivated by a tiny edge case related to protecting partial updates after two clients simultaneously try a PUT. However, I can't remember if it was when one succeeds and one fails, or one or both responses are lost and they try again, or something like that, nor can I remember what difference was made by not sending ETag. Hence, I intend to remove the requirement as being too obscure to implement in a testable way.

Revised text:
. A recipient MUST ignore the If-Unmodified-Since header field if the  <ins> received field value is not a valid HTTP-date (including when the field value appears to be a list of dates). </ins> A recipient MUST interpret an If-Unmodified-Since field value's timestamp in terms of the origin server's clock.