Hi,
Glass optics expert and model S owner here...
What you are experiencing is the "double image" effect. Most of the light passes through the windshield just being refracted twice at each glass interface. That's why you can see through the windshield. Some light is reflected on either the outer or inner interface. That's why you see reflection on the glass. But a smaller part enters in the glass, reflects twice inside the glass, and exits the glass. This also contributes to transmission, but as the glass is slightly curved, the resulting image is slightly offset vs the main image. It is also much fainter, so you only see it when there is a bright object on a dark background, ie night conditions.
This effect exists on any windshield, but:
- Is somewhat compensated by some manufacturing tricks, depending on the windshield shape
- Is very dependent on viewpoint. Especially, the compensation generally only works at an average driver height.
- May vary slightly within and between production batches
- May vary between suppliers.
I did not notice it upon delivery, but I did not drive much by night as I had the car in spring. I quickly had some "clicking" noise issues coming from the windshield when accelerating/braking, after a few visits to the SC they finally changed it in autumn. After this I drove by night and noticed the double reflection a lot, but I'm not sure if the first windshield had it or not.
The problem with this effect is, as soon as you noticed it, you don't see anything else... But it's potentially there on any car brand (especially since the same few glass manufacturers supply every car brand). It will be hard to have the windshield replaced under warranty for this.