I'm guessing you have different events for different sprites?
Mouse -> On Sprite1 clicked -> (start dragging Sprite1)
Mouse -> On Sprite2 clicked -> (start dragging Sprite2)
Just create a flag (a global variable) isDragging. Set isDragging=1 when a sprite is clicked, reset to 0 when it's dropped.
Add a second condition to all these events:
Or if all these draggable sprites are in a family, and you have just one event (Mouse->On DraggableFamily clicked), add DraggableFamily->Pick Top Instance as a sub-event.