[About XDA developers] V@no:
I could reproduce it in different browsers (not 100% though), this means it's not plugin related

[EDIT]

Even though I didn't get the double post issue this time, but here is the detailed information from Live HTTP Headers extension for Firefox: (I replaced some sensitive data with <removed> tag)

http://xdaforums.com/newreply.php?do=postreply&t=2593689

POST /newreply.php?do=postreply&t=2593689 HTTP/1.1
Host: xdaforums.com
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:26.0) Gecko/20100101 Firefox/26.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-us,en;q=0.7,ru;q=0.3
Accept-Encoding: gzip, deflate
DNT: 1
X-Requested-With: XMLHttpRequest
Content-Type: application/x-www-form-urlencoded; charset=UTF-8
Referer: http://xdaforums.com/showthread.php?p=49241748
Content-Length: 404
Cookie: bblastvisit=1388113345; bblastactivity=0; bbuserid=620411; bbpassword=<removed>; compactview=0; tab=top; vbulletin_collapse=forumbit_2540%0AThanks_47472268%0AThanks_47561416; cX_P=1377149517077967602642; xda_wikiUserID=317811; xda_wikiUserName=V%40no; xda_wikiToken=<removed>; __ktp=<removed>; visited=1; vbulletin_userlist_hide_avatars_incomingreqs=0; vbulletin_userlist_hide_avatars_buddylist=0; bbsessionhash=<removed>; cX_S=13891938285901641944020; xdaloggedin=1
Connection: keep-alive
Pragma: no-cache
Cache-Control: no-cache
securitytoken=<removed>&ajax=1&ajax_lastpost=1389197237&message=I%20can%20reproduce%20it%20different%20browsers%2C%20this%20means%20it%27s%20not%20plugin%20related&wysiwyg=0&styleid=0&signature=1&fromquickreply=1&s=&securitytoken=<removed>&do=postreply&t=2593689&p=49241748&specifiedpost=1&parseurl=1&loggedinuser=620411

HTTP/1.1 200 OK
Server: nginx
Date: Wed, 08 Jan 2014 16:10:06 GMT
Content-Type: text/xml; charset=windows-1252
Transfer-Encoding: chunked
Connection: keep-alive
Vary: Accept-Encoding
Cache-Control: private
Pragma: private
X-UA-Compatible: IE=7
Set-Cookie: xdaloggedin=1; expires=Wed, 08-Jan-2014 16:25:06 GMT; path=/
Set-Cookie: visited=1; expires=Sun, 04-May-2014 09:56:46 GMT; path=/
XDA-No-Cache: 1
Content-Encoding: gzip
----------------------------------------------------------

http://xdaforums.com/newreply.php?do=postreply&t=2593689

POST /newreply.php?do=postreply&t=2593689 HTTP/1.1
Host: xdaforums.com
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:26.0) Gecko/20100101 Firefox/26.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-us,en;q=0.7,ru;q=0.3
Accept-Encoding: gzip, deflate
DNT: 1
Referer: http://xdaforums.com/showthread.php?p=49241748
Cookie: bblastvisit=1388113345; bblastactivity=0; bbuserid=620411; bbpassword=<removed>; compactview=0; tab=top; vbulletin_collapse=forumbit_2540%0AThanks_47472268%0AThanks_47561416; cX_P=1377149517077967602642; xda_wikiUserID=317811; xda_wikiUserName=V%40no; xda_wikiToken=<removed>; __ktp=<removed>; visited=1; vbulletin_userlist_hide_avatars_incomingreqs=0; vbulletin_userlist_hide_avatars_buddylist=0; bbsessionhash=<removed>; cX_S=13891938285901641944020; xdaloggedin=1
Connection: keep-alive
Content-Type: application/x-www-form-urlencoded
Content-Length: 299
message=I+can+reproduce+it+different+browsers%2C+this+means+it%27s+not+plugin+related&wysiwyg=0&styleid=0&signature=1&fromquickreply=1&s=&securitytoken=<removed>&do=postreply&t=2593689&p=49241748&specifiedpost=1&parseurl=1&loggedinuser=620411&ajaxqrfailed=1

HTTP/1.1 302 Moved Temporarily
Server: nginx
Date: Wed, 08 Jan 2014 16:10:07 GMT
Content-Type: text/html; charset=ISO-8859-1
Transfer-Encoding: chunked
Connection: keep-alive
Cache-Control: private
Pragma: private
X-UA-Compatible: IE=7
Set-Cookie: xdaloggedin=1; expires=Wed, 08-Jan-2014 16:25:07 GMT; path=/
Set-Cookie: visited=1; expires=Sun, 04-May-2014 09:56:47 GMT; path=/
XDA-No-Cache: 1
Location: http://xdaforums.com/showthread.php?t=2593689&goto=newpost