This seems to be a new question, so I have moved this into its own topic.
Do you want to prevent nodes occupying the same location during a drag? Or do you wznt to prevent that only upon a drop? And should a “bad” drop cancel the whole drag-and-drop or just fix those coincident nodes by moving them somewhere?
For the latter case, implement an “ExternalObjectsDropped” DiagramEvent listener that iterates over the newly dropped nodes and checks each one for occupying a cell containing any other nodes. Then you can either cancel the whole drag-and-drop by calling e.diagram.currentTool.doCancel(), or you can move those problem nodes to acceptable locations.