Hi Martin

I was able to replicate this using multichoice in a quiz where I got the option to redo quiz even if the answer was right 

The option for Allow redo within the attempt inline help says

If enabled, when learners have finished attempting particular question, they will see a Redo question button. This allows them to attempt another version of the same question (or another question from the question bank in case of the Random question), without having to submit the entire quiz attempt and start another one. This option is mainly useful for practice quizzes.

This doesn't say whether the answer needs to be right to redo it  and suggests that this option is usually selected when there is another question rather than the same question.

The help doc needs to be lined up with what the inline help says.

Regards