One of the things I really despise about Duolingo is having to go back and strengthen previous lessons/exercises. I can understand having to strengthen one or two a week but it reaches a point that you have to redo almost everything. It does not feel like any progress is being made when you have to redo and redo and redo and redo....!!!!!
Duolingo's patterns are designed to predict when you need to redo them to get them into your long term memory. If you haven't done any Duolingo in a while, it'll make you redo some. If you feel like you already know that particular topic very well, you can choose to not redo it. Although I would still recommend redoing them as often as needed, you'll remember much better.