Im under impression that any stun halves the duration of next.
So if ure a tauren and stomp someone for those 2 seconds, any next stun will be 50% duration. etc
edit: you could probably find a list of spells that share diminishing category, for example shaman hexes someone for full duration, poly on that target would be half, 5 secs only.
diminishing expires after 15 seconds, so next stun would be again full duration
download either (test which one suits you better) so your stuns can always be used to full
http://wow.curse.com/downloads/wow-a...ngreturns.aspx
http://wow.curse.com/downloads/wow-a...ladiminish.asp