\gradeimport_csv_load_data::check_user_exists() can unreliably fetch another user's ID

Create students s1 and s2. Give them the same ID number. Enrol them in a course with some activities (e.g. assignment) Go to the course homepage. Navigate to " Grades / Grade administration / Grader report " Click on the Export tab. Under the Export tab, click on the Plain text file tab. Click Download. Open the downloaded CSV file using a text editor and set a grade for s2's assignment. Save the edited CSV file. Back on the browser, click on the Import tab. Drag and drop the CSV file to the File picker. Click " Upload grades " Under " Identify user by ", set the following: Map from ID number Map to ID number Under " Grade item mappings ", map the assignment accordingly. Click Upload grades. Confirm that you get an error that multiple users were found. Confirm that the import fails.