Technically a mirage contains an inverted or duplicated image. This is not a mirage, it's a type of refraction phenomenon.we can decide when the island transforms from a mirage to actual.
It's a gradual transition, and not one you would be able to pinpoint.
You could actually show this transition to a degree by taking images from those steps. One per step, with Matinicus centered. You'd see what refraction is doing.