Check if an element has event listener on it. No jQuery [duplicate]

Answer 1

You don't need to. Just slap it on there as many times as you want and as often as you want. MDN explains identical event listeners:

If multiple identical EventListeners are registered on the same EventTarget with the same parameters, the duplicate instances are discarded. They do not cause the EventListener to be called twice, and they do not need to be removed manually with the removeEventListener method.