As stated in Nintex Forms - Known Limitations and Behaviors: "Enforce Unique values not enforced in list forms".
I am wondering though whether someone might have found a workaround for this shortcoming.
I am designing a form where users can request a new site, and a certain field of my form has to be unique because the URL of the to-be-created-site will be based on the field (what my workflow is taking care of).
Solved! Go to Solution.
I found this an interesting idea, so I did a little digging. The Nintex form does not validate unique values like a SP form will pre-save, but SharePoint still controls the unique values by throwing out the update to that field. So I setup a list with unique values on a single line of text field and enabled indexing on it. In a SP form, I click save with a duplicate value in the field and I see a validation that it is not unique in red next to the field. I create a Nintex form and do not make any other changes and publish the form. When I put in a NON duplicate value in the field, I see my changes in the new item. If I put in a duplicate value in a new item, the field in question does not have a value saved in it, but the other fields do get updated. So it seems like SharePoint rules win in the end and tosses out the value when a duplicate. But you do not see a pre-save validation.
So the unique value enforcement seems to be working. Just not the way you may need it to as a validation error pre-save.
The only way I see this working is using a technique from InfoPath. Creating a data source to the field to test against, then see if the value entered is contained in the data source. But i'm not sure how you would do that easily in a Nintex Form.
Maybe others have found better solutions