In your doDragStart handler, add a unique identifier to your DragSource. (ie. dragSource.addData(someID,"id"). When you accept the drag, store that id. If you encounter that id again, don't accept the drop.
OK I will try that thanks.
So I think that last answer would of worked but with the number of Icons I have this will take all year.
Does anyone know how I can just tell the icons where to drop on a large canvas? For example:
When you drop a generator Icon, it goes to an exact spot on the canvas, then when you drop another generator icon, it goes to a spot below it etc.
You icons are getting their data from somewhere, right? If the icon URL is unique, then you can use that. It is it not, then you can generate a unique ID for each icon. You can use mx.utils.UIDUtil.createUID to dynamically generate an id for you.