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.

So I know of a SPService js function SPRequireUnique. I tried to implement it into a Nintex form but was unsuccessful. jQuery Library for SharePoint Web Services - Documentation The issue was it cannot find the form filler input box to validate for. So maybe there is a way with JavaScript, but it will take a lot more work.

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

‌ how did you resolve this in the end?

Hey Cassy, 

There was no real solution in my form in the end (guess with some JavaScript you could, but...). I solved it in in my workflow. First check is whether there already exists a site with the same URL. If yes, a mail is sent to the requester to re-enter the details in the form, with the message that the URL is already taken. 

Regards, 

Glenda