Students are allowed to submit solutions multiple times until the deadline of the assignment has passed. However, Nuvolos only allows you to grade the latest submission coming from the same instance, and by default, only these submission are shown on the Handin tab. To inspect non-latest handins, untick the "Show only gradeable handins" checkbox on the Handin tab.
Also, the name of each submission may be choosen freely by the students, to support using custom identifiers (e.g. student ID number, etc.). Nuvolos will group submissions coming from the same instance as different versions coming from the same entity, and only the latest submission will be gradeable, even if there were different submissions in the same instance by different users under different names.