If a student submits an assignment, they can go back to that submission by re-opening the assignment. They will see the current submission as well as a link to resubmit (as long as the assignment isn't closed).
Have I understood i right, if they resubmit they get an empty RTE write in?
And thanks for your answer.
Yes, that's correct. As the instructor, you can choose which submission to look at if there are multiple submissions for a student.
Thank you again! Is there any other tool, in Canvas, that let the students both hand in and be able to continue write in the samer text?
Not that I know of. When I have students doing longer assignments, they write in a Google Doc and submit checkpoints as a URL rather than using the RTE. I can see their progress and they don't lose any work. Those checkpoints don't necessarily have to be separate assignments, either. They can continue to resubmit the same one.
Thank you, I think Google drive/Office365 seems to be the way to go.