Armor Duplication Bug on MC 1.4.2

I tried to fool my friend with the invisibility potion and rapidly clicked on my armour in the armour slot and it suddenly duplicated.
I was in creative mode playing on a server - Craftersland

I have a video i posted on youtube, here is the link
http://youtu.be/dQIMEfl7G-o

Watch the video until the end. The first part I failed a little bit.