If they are lectures clashing it probably won't be a problem. Lectures are generally recorded and attendance is not taken so there is no real way to tell who has and hasn't attended. The only time it would be an issue is if you had an assessment during a lecture, then you would have to attend at your enrolled time.
They generally will not add additional lectures, only tutorials/practicals.
I would personally attend one class in week 1 and the other in week 2 and see which you feel is more important to attend the physical lecture. Then complete the other lecture online if you need to.
I'd argue the vast majority of people don't attend lectures. In the first few and final weeks they can be busy, but in the middle of semester attendance is quite low. You won't be penalised for using the online lecture.
