[nicoledb]

If you know beforehand that you would always mark the weekend-tasks as done before a certain time on Monday, you could set the start time for the repeating tasks a bit later in the day, like 10 am. That way, you're marking the 'old' items complete, and they show up again at 10 am to be done today.

Another way of looking at it is that you can't mark them as done today, because you haven't done them today yet.

And another option is what you suggested in your original post: when a second instance becomes active, previous instances could/should be dropped. If you didn't brush your teeth yesterday it doesn't make sense to brush them twice in a row today.