Prevent user from returning drop item to original position

I created a slide where a variable increases by a specific amount when objects are dropped on their targets. It works great, but the user is able to return the drop item to its original spot, then drop it back on the target--which is causing the total to increase (basically, it is double counting.) How do I prevent a user from returning a correctly dropped item back to its original position? I feel like this should be easy but can't seem to find the setting. The behavior can be viewed in the attached screencast.

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