When you are defining the field click the 'Options' button and then the 'Validation' tab and check the 'unique value' option. You might want to untick 'Allow user to overide during data entry' as well.
Hope this helps.
Click, on file, manage --> database
Select the field you want to make unique and click options
In the options menu select the validation tab and check the box where it says Unique Value
I have done this with a date field.
This was a scripted NEW RECORD. I use a very abbreviated layout to get the date field in question, using a global field. Then I did a FIND on that date. If not found, it created a new record pasting the date into the record. Before terminating the script I moved the cursor to the next field.
If the record was found, it was displayed; terminating the script.
Is there a way to do this with multiple fields? For example, in a contacts database you may need to allow two John Smith's to exist, but not two John Smith's at the same company. Can you set parameters that look at multiple fields for validation?