Does the order of exercises change when I redo a lesson?
By "redo" I mean tapping the "redo" button of a lesson. Does duo shuffle the exercises after I complete a lesson?
The "skill strengthen" button sometimes adds NEW words, where the lesson button (with described vocabulary) have failed to present those words.
As the sentences are pulled from a global database, there may be more than just one sentence per word.
I am not an expert in this topic. Matfran2001 is.
Your question is IMO very good, if the words and sentences are very hardwired to the lessons 1 to 10 or how the "poll from sentence DB for word X" exactly works behind the scenes.
I just know, that when you click on "Strengthen", but always new words can popup, which you have never seen, even you may have completed your Duo tree.
The good thing about re-doing all 10 lessons for a grammar skill would be, that you have MORE training, than just to click 2-3 times on the strengthen button :-)
The sentences are usually the same, but sometimes they're not. In some courses there seem to be sentences in the lessons that never come up in strengthening sessions, so I definitely incorporate redoing of lessons into my general Duolingo habits.
EDIT: This answer refers to the website; I don't think I've redone enough lessons / paid enough attention on the app to have much of an opinion. The lessons there often have more sentences, so it's harder to keep track.
Sometimes it the order does change, or new sentences appear, although I don't know what governs this. On the app, I find it tends to vary more; in general, however, it usually remains the same, in my experience. There might well be A/B-tests affecting this, too, so your situation might differ from mine.
Clicking the practise button within a skill is a better way to get shuffled sentences (although, obviously these will not be confined to the material of a single lesson).