UserAgent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Ubuntu Chromium/59.0.3071.109 Chrome/59.0.3071.109 Safari/537.36

Example URL:

Steps to reproduce the problem:
1. Make a web server return a response with the Last-Modified header being present twice, e.g. with this response in chrome://net-internals/#events:

t=2463 [st=1]        HTTP_TRANSACTION_READ_RESPONSE_HEADERS
                     --> HTTP/1.1 200 OK
                         Last-Modified: Fri, 21 Jul 2017 15:01:56 GMT
                         Content-Length: 18028
                         Accept-Ranges: bytes
                         Date: Sat, 22 Jul 2017 17:21:14 GMT
                         Server: Warp/3.2.12
                         Content-Type: application/font-woff2
                         Last-Modified: Fri, 21 Jul 2017 15:01:56 GMT

2. Refresh that page
3. You'll see that Chromium sends an invalid If-Modified-Since entry:

t=55809 [st=0]        HTTP_TRANSACTION_SEND_REQUEST_HEADERS
                      --> GET /static/myfont.woff2 HTTP/1.1
                          Host: localhost:8000
                          Connection: keep-alive
                          Origin: http://localhost:8000
                          If-Modified-Since: Fri, 21 Jul 2017 15:01:56 GMT, Fri, 21 Jul 2017 15:01:56 GMT
                          User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Ubuntu Chromium/59.0.3071.109 Chrome/59.0.3071.109 Safari/537.36
                          Accept: */*
                          Referer: http://localhost:8000
                          Accept-Encoding: gzip, deflate, br
                          Accept-Language: en-US,en;q=0.8
                          Cookie: [71 bytes were stripped]

Notice the entry:

  If-Modified-Since: Fri, 21 Jul 2017 15:01:56 GMT, Fri, 21 Jul 2017 15:01:56 GMT

What is the expected behavior?
Chromium sends only one, valid, not-comma-separated If-Modified-Since value, or does anything else but not send two dates comma-separated.

What went wrong?
According to https://stackoverflow.com/questions/4371328/are-duplicate-http-response-headers-acceptable:

"Multiple headers with the same name is ok if the entire field-value is defined as a comma-separated list of values."

Neither Last-Modified nor If-Modified-Since permit comma-separated values:

https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.29
https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.25

Independent of the fact that the web server returned an illegal response here (as Last-Modified can't be present twice), Chromium should not react to that with sending an invalid header itself (dates separated by comma don't seem to be permitted, and don't seem to make any sense to me).

I suspect that there's some logic in Chromium that automatically combines headers. I suspect it does so even when the result would be invalid.

Did this work before? N/A 

Chrome version: 59.0.3071.109  Channel: stable
OS Version: Ubuntu 16.04
Flash Version: