You should be able to resume a failed lesson later (without starting over) if Duolingo made mistake(s) grading it.

[amichail]

This would also give people motivation to report answers that they believe are correct that Duolingo graded as wrong (provided that they are well informed of this motivation).

[michaeldressner]

You are forgetting the people who will report wrong answers unjustly... Factoring in the time it takes for the staff to review the question (this is not instantly) I think it would take less time just to redo the lesson..

[amichail]

You might wait a day before redoing the lesson just in case your reported answers are judged correct by the staff.