If I was to return 10 marks from a class of 30, then a month later return the other 20 marks, would the first batch of 10 marks be overwritten, or (wrongly) show a 2^nd attempt?

The marks return system only returns a mark as a second attempt where there is already a different return with a mark for that student.

The recommended way to do this is:

1. Create a single marks return in the system
2. Add the 10 marks and send them to student records
3. Later on, edit the same marks return and add the other 20 marks and send them to student records

When a return is edited and sent to student records, only the marks which have changed are transmitted so the original 10 marks will be unaffected.

See also Batched Returns