Slow down Duolingo!
I am learning German and whenever I try to review some of the past subjects, new words constantly show up, even though I should only be reviewing what I've already learned. There is nothing wrong with learning new words, but the problem is that the way you learn new words is by learning a few at a time and having to use them in sentences over and over until they stick. But these new words come and go so randomly that I can't remember them. Also, the worst part about it is that Duolingo then adds the words to the number of words I apparently know, after I used it once. ONCE.
This bring me to my next point. How easily it is to "master" a new set of words. I do a section in "Dative Case", fail over and over and finally get through it with zero hearts left and it says I have now "mastered" 7 new words. See, I'm the kind of person who hates taking credit or being given credit for something that I didn't do, especially if it is something good. I just barely learned those words, I failed multiple times and I made mistakes and Duolingo thinks I mastered them. I know I just have to keep reviewing the sections until I feel I personally mastered them, but at the same time this can easily be fixed.
I agree Duolingo can move very quickly and I know when I was starting French and doing the present tense verbs each unit introduced more and more verbs and may have only showed them once and considered it "learned" if I was exposed to just one form of its conjugation one time. That being said, looking at the vocabulary algorithm page most of the verbs that I click on, even the ones I feel I know pretty well, have a total "word strength" between 1-2 bars meaning that although my lessons mostly appear golden the system is well aware that I need to see those verbs many more times to have truly mastered them. So I think that when you first learn new words the system logs that you have only seen it one time so it marks it as "mastered", but the decay rate will probably be a lot faster than words that you have seen maybe 5-10x and consistently answered correctly hopefully ensuring that you see those you don't know as well more often. But I think reviewing freshly learned sessions as you are currently doing is helpful sense, as pointed out often in the discussions, there are lots of sentences to learn for any one topic and doing the lesson several times will expose you to those words in different ways.
One thing that can get annoying though, like you mention, is all the new topics showing up with the old topics you are trying to review. It is helpful in the sense that it is reinforcing the new topics and showing you how they can fit in with what you have already learned, but it can make reviewing older topics much more difficult.
Keep a notebook with you. Write down every new word that comes your way. Review and practice with timed tests until you feel comfortable to move forward. They didn't design these so you could just blow through them and finish in a month.
The mastering words phrasing is misleading though, I agree. I believe over time the strength of them diminishes if you haven't seen it in a while.
THIS: "They didn't design these so you could just blow through them and finish in a month."
And also: Just because you can do something doesn't mean you should...
I find it kind of odd that quite a few people seem to think Duolingo goes too fast - you can progress through the lessons as fast or as slow as you want to. Perhaps the issue is that it is human nature to want to keep moving on to new "exciting" stuff... even though that's not necessarily the best way to learn. (Research has repeatedly shown that spaced repetition is the best way to learn a language (and many other things))
I use the review function a lot. Every day I log in actually - before doing new lessons. Sometimes I use the review all my skills button, usually I just pick one or two skills and review them. I've been doing this since I started because I think it'll help me build a more solid grounding of the language. I probably spend at least as much time reviewing as I do learning new lessons.
You can re-do an old lesson by clicking on the lesson rather than the review button. I don't know if that give you the exact same lesson as when you first did it but it might be worth a try. You can peek while reviewing too. I heard somewhere that peeking makes the word show up in your review practice more often, but that's hardly a problem - I wouldn't have peeked at it if it was already in my memory, would I?
I've only noticed a new word while reviewing once or twice by the way - maybe that's just me. Unlike most of the commentators here though, I don't take notes (because I can't be bothered). I think regular reviewing is enough for me.
Like many other resources learning a subject I've found Duolingo's not able to cover every single aspect to meet individual needs. If you don't think it is giving you enough focused practice and it seems too fast, make flash cards - find a flash card website, read about the grammar as a supplement, take notes on words - I take notes when I mess up, whatever you need to slow it down. On the other hand, if it feels like a snail's pace and this is all review maybe hitting the unlocks to reach the end lessons, playing with immersion, and reading a book is more appropriate.
Usually I take notes as I go through the lesson on concepts, accents, and vocabulary, and this helps me remember it better. It's not complicated, all you need is a few bullet points for the key concepts and a chart to write in vocab words. Also, if I don't feel comfortable with something, I keep practicing that particular section over and over until I think I'm ready to move on. It is vital that you understand and are proficient in everything you've been taught so far when you start a new section.
Another good thing to do that will strengthen your vocabulary is to try and speak things in German around friends/family members. I am learning Spanish, and if I want someone to get me a glass of water, I'll tell them in Spanish (Yo quiero un vaso de agua, por favor) and then tell them what I said in English (I want a cup of water, please). They say that you learn things more easily and better retain information if you can teach it to someone else.
Answering questions in the discussions (forum or on the individual sentences) can also help you retain it better again because you're teaching someone else.
I always double check any answer I give if I'm not 100% sure, so this forces me to look up grammar websites and/or my Italian dictionary more often than I would if I never tried to answer other people's questions.