You might know or not, but the twelfth fret is the point where notes start to repeat. From the twelfth fret onwards, all notes will repeat while being one octave higher. So, the twelfth fret is the same as an open string, but one octave higher, and some go with thirteenth, fourteenth, and other frets. This is why the twelfth fret has two dots on it.
Also, if you have a 24+ fret guitar, you will notice two dots on the 24th fret. The science is the same there too. The notes will start to repeat, but they will be two octaves higher this time.