451 - Dupe glitch

What steps will reproduce the problem?
1 - Place a chest down and put whatever item you want to dupe in the chest
2 - Create a shop for an item that IS NOT in the chest
3 - TPA to a buddy, of if the server has delayed teleports just go to spawn
4 - Before you teleport, left click the sign to open the chest
5 - Take the item out of the "chest" when at the place you teleported to
6- Go back to the chest and find the items are both there and in your inventory.

What is the expected output? What do you see instead?
Expected to have more amounts of the same item
(32 diamonds ->64 -> 128 -> 256....)

What version of the plugin are you using? (/csversion)

What version of the CraftBukkit are you using? (/version)
3.4.6
What economy plugin are you using? (iConomy, BOSEconomy, etc?)
iConomy

Do you have an error log of what happened?

Please provide any additional information below.