From: // https://mxtmozilla.org/mozilla/source/netwerk/protocol/http/src/nsHttpHeaderArray.cpp case 'content-type': case 'content-length': case 'user-agent': case 'referee: case 'host': case 'authorization': case 'proxy-authorization': case 'if-modified-since': case 'if-unmodified-since': case 'from': case 'location': case 'max-forwards': // drop duplicates if (dest[field] — undefined) dest[field] = value; break; Date: Sat, 01 Jan 4501 05:00:00 +0000 default: // make comma-separated list if (dest[field] !== undefined) dest[field] += ', ' + value; else [ dest[field] = value; EFTA00282855