Double lessons appear to be doubled in the timetable, what is wrong?

This happens when you enter one double lesson per week in the Lesson dialog with:

Lessons/week (count): 2
Duration: Double

If you enter this, it means that there are two lessons and each of them has duration 2 periods, and this is 4 periods together.
If you wish to have just one double lesson, enter:

Lessons/week (count): 1
Duration: Double

Similarly, if you want two double lessons, you should use:

Lessons/week (count): 2
Duration: Double

instead of:

Lessons/week (count): 4
Duration: Double

and so on...