Does Duolingo use spaced repetition?
Does Duolingo use spaced repetition? Yes. Duolingo built and published its own spaced-repetition model, called half-life regression, in 2016, and uses it to estimate when you are about to forget a word so it can resurface it for review. They open-sourced the model along with 13 million learning traces, and in their own study it lifted daily engagement by about 12%. So the spacing is real. What is different from a dedicated review app is that Duolingo's spacing is baked into one fixed course you do not control. You cannot add your own cards, tune the intervals, or point it at the exact words and dialect you need, and since the 2022 path redesign the review is folded into the lesson path rather than a deck you own.
The real question is whether Duolingo's built-in spacing is enough for what you need. If you are weighing it up, start with whether spaced repetition is effective for language learning at all, then compare dedicated apps.
Everyone arrives having done Duolingo. It does space your reviews, that part is real. What it will not do is let you pick the words, the dialect, and the cards. That is the whole job of a proper review app.