I'm using xfce4-panel (but it also happens in tint2). If I have my laptop plugged in and I remove the battery, my system tray will have two plug icons in it. When I replace the battery, one icon will be a battery and the other stays a plug (I think. I don't have my computer with me right now).
This has been fixed in git master. The panel-plugin groups multiple devices and shows them in a menu.