This most likely means they have registered their license key for the material under two different accounts*:
* Account #1 was registered under one email and Account # 2 was registered under a different email.
This created a conflict between two accounts as student has been completing some Online Practice activities in Account #1 and then logged in to their Account #2, which obviously did not have stored progress they have made in Account #1.
Merge two accounts, so their progress can be consolidated. To have your students’ accounts merged, you need to submit a case to firstname.lastname@example.org.