Have you tried using the Unique values setting on the validation tab in Field Options for your field?
Currently the field is set up as an "Edit Box" (I believe) and is not giving me a "Field Option", only Field/Control containing Setup, Behavior, and Borders.
Open Manage | Database | Fields and find your field there.
Double Click it.
Click the validation tab in the field options dialog that now appears and select the unique values field option.
Thank you! Works perfectly.