"The changes you requested to the table were not successful because they would create duplicate values in the index, primary key, or relationship. Change the data in the field or fields that contain duplicate data, remove the index, or redefine the index to allow for duplicate entries."
... you may have a corrupted index (especially if the database worked before without any problems).
Basic Fix: Tools>Database Utilities>Compact and Repair
If this doesn't fix it:
Create a new blank database and import the database into it. Run Compact and repair.
If the problem continues, you may have to do a little troubleshooting. If you know that the error message only pops up when you add a record on this form, recreate any queries running behind the form from scratch. If it doesn't fix the problem, you might try recreating the form.
Microsoft also recommends deleting and recreating the relationships when a database is corrupted.