Duplicate Grade Items Created when Syncing Grades on D2L Assignments with Auto-Create Grade Items Enabled

## Bug report form
### Steps to reproduce
1. Create a Hypothesis assignment in D2L and do not use Max Points or manually add a grade item. Let the auto-create grade items feature of D2L create the grade item.
2. Enable Auto grading.
3. Have multiple students annotate it.
4. Sync Grades from the dashboard.

### Expected behaviour

- A grade item will be associated with the assignment.
- A single column corresponding to the assignment will be created in the gradebook

### Actual behaviour
- multiple but identical grade items were associated with the assignment
- multiple columns in the gradebook were created
- the number of columns and grade items created is equal to the number of grades synced.

### Browser/system information
Chrome 140
Mac OS Tahoe v26

### Additional details

Issue does not happen when [Max Points](https://web.hypothes.is/help/max-points-in-hypothesis-enabled-readings/) feature is used.
Hubspot ticket: https://app.hubspot.com/contacts/6291320/objects/0-5/views/14722589/list
D2L course: https://aunltd.brightspacedemo.com/d2l/le/content/7128/Home