How would I spawn a single entity when current entity is looked at

How would I make a single entity spawn when the current entity is looked at and basically replace the old entity with the newly spawned one? I was able to make an entity despawn when looked at but when I try to do the same thing but with spawning an entity it spawns like millions of the entity I want to spawn and I only want one entity to spawn when the current entity is looked at. Anyone able to help out will be appreciated a ton, thanks!