I just tested with the following, which worked as expected:
1) created a new 40-character String when personalizing a form
2) Personalize Dictionary on that new field, add the Unique field to the Dictionary form, set it to true, and Update
3) Enter a value for that field in one record in my table, and Update
4) Enter the same value for that field in another record in my table, and attempt to Update

After clicking Update on the second record, I got the expected error: "Unique Key violation detected by database" and the form submission was cancelled.