Repeat a lesson impossible.
Arabic is so different from English, that we should be able to repeat a lesson immediately. Instead we are pushed along to the next lesson before we are ready.
Go to https://duome.eu/hugadale/progress (after having switched to the respective language within duolingo), pick the lesson and click the "practice"-button (bar-bell symbol).
Or you can directly go to the tree (browser version), pick the lesson and click the "practice"-button of the lesson (not the bar-bell symbol this time!).