I used to work for SOE(r) Sony Online Entertainment. As a volunteer Guide. And for other online game platforms. And was offered GM position's even. Anyhow, I found and fixed many bugs etc over the years. This one is on your minecraft game this last month and havent told anyone of it. Simply users disconnect then reconnect to a server. The user see's there other self waiting to time out. All they do then is attack the mirror self that dies and collects all that inventory. Basically doubling the loot they had on them self. I am certain it is a fairly simple fix that you can attend to. But wanted to make you aware of this as people who host would not want this exploit getting out or out of hand as it would ruin economies in servers ultimately. I hope you found this helpful. You could possibly fix this with a login timeout patch. So they half to wait till the server see's that character has disconnected fully before re-login.
~Pixelkungfu