You actually have three moons there Tom. Sometimes with such a bright object you can get significant internal reflections in the lens to produce a secondary spot like this.
That lens has 17 elements in 12 groups and although the internals are coated to reduce reflections it is not possible to totally avoid them. The colour shift of the prominant greenish spot makes me think that is an internal reflection of the image. The whitish one is more likely, as you suggest, double glass 'refraction' effect but depending upon the angle that this was taken, it could also be an internal reflection.
It is interesting to play with shots like this to observe the various effects produced.
Shooting through double glass, well that is a challenge. ~ Regards melbob
