Thank you all for the help.
Although the addition of a variable to update and adding if statement to drop function works, it also causes all the drop targets to become non-drop targets.
Thanks Newbie for this code:
if(ev.currentTarget.innerHTML == "") {
}
Although I have not researched the innerHTML property yet, that did the trick. So once you drop on a target, that target's innerHTML property get set to some value, and ultimately blocking other objects from being dropped on that target again.