If the student has 2 different accounts, with different usernames, you can merge their duplicate accounts. This way, they have 1 set of login credentials and all their assignment on one account.
Read here about merging: https://support.commonlit.org/hc/en-us/articles/115000758734
If the student appears twice on your roster with the same exact username (and therefore the same name and email), please contact firstname.lastname@example.org. Do not alter the account. If you attempt to merge the two accounts or delete one of the accounts from your class, you will remove both accounts from your class.