It where a bad choice to set the ALT key as key for copy entities.
In most programs ALT is used for many other functions like cam around object.
The result is , because high fidelity use again some strange keyboard layout, that you press the ALT key when you want to CAM. But as result you start to duplicate.
Something as SHIFT+D would make more sense.
It happens to many times that yopu press ALT to cam around the object. Like you do in Secondlife. Result is duplication of objects. And still not a good way to cam around object.
Lucky it’s easy to see that you now duplicate a light.