Students can end up with multiple accounts because of typing errors when registering, multiple valid email addresses, or a change in the school domain.
If a student claims to have submitted work but it cannot be found in the Markbook, look for a second account. The student might have signed up with the wrong email address, or registered with multiple addresses in various subjects.
Do not remove either account from the subject.
Instead, contact Stile's Support team to merge the accounts together. All of the students' work will be preserved in a single account.