Yes, this is a bug. No, this is not a critique or an idea. This needs fixing.
Currently, it is possible to have more than one client open at a time, making it very easy to add alts to you kin list, and swap items with them.
This is a bug.
The easy fix: When you log in, have it send the client computer's MAC address to the server. When they log out, remove that MAC address from the list of online MACs. If someone attemtps to log in with a MAC address already in use, deny access.
Voi-fuckin-la, problem fixed.
Edit: Just a note: the only two ways around this are
multiple computers (which should be allowed anyway, in the case of multiple people on a LAN),
and multiple network adapters in a single computer. This is pretty much unbeatable, unless you scan all MACs on the computer and send them all.