We don't love detail, but if we don't get that detail, we often cannot offer effective help. In your case, there's a simple solution.
Go to Manage | Database | Fields
Find your calculation field and change the field type to "Text".
Double click it to open Options.
On the auto-enter tab, select the calculation option and also clear the "do not replace existing values" check box. (The calculation you defined for this field will automatically appear in the specify calculation dialog that opens when you select the Calculation button.)
Now you can click over to Validation and specify unique values as the validation field option for this field to limit the value in this field to unique values.
Thank you for the Acme hammer, and pointing out the obvious. i generally never think of text as a calculation point to start from. I will play with this approach and see where it leads to.
Peter Kelley - St. Paul, MN USA
Please note that I had to assume a detail not explicitly stated in your original post:
That your calculated text field and the fields E1, E2, E3, E4, E5 and E6 are all defined in the same table. This method will not work if this is not the case.