It's definitely not "just" spaced repetition in the way that Anki is (asking you how difficult it was to recall the answer and choosing to ask you again at some time "t + now" in the future, where "t" is determined by your answer). As far as I can tell, as you're working through a tree in Duolingo, it tries to sprinkle in words that you've seen a few checkpoints ago, but it doesn't seem to do so dynamically based on demonstrated mastery or lack thereof of the words sprinkled back in.