Is there a way to repeat lessons I have already completed?
It's not currently possible to repeat an individual lesson in a skill unless you use a Windows 10 app or an old and outdated Android app (e.g. 3.56.1).
You can, however, practice the whole skill. The practice button is available either when you get to the last crown in the skill or you can access it via your (unofficial) progress page: https://www.duome.eu/Gri7f/progress
Like 40ShadesOfGreen said, it's not currently possible unless you have old devices - or if you haven't updated the app. You can practice the whole skill though. You can do it how 40ShadesOfGreen said, or you can click on the Practice button, which mixes your most recent lessons with other lessons that you're slightly behind in.