If a People picker column is configured to only allow a single user/group, then it is still possible to search and add multiple users into a Nintex Forms, People picker control. 

The validation is triggered when the Nintex Form is saved, as opposed to how the People Picker legacy control works, where it would perform a validation after the first user/group has been added.

This has been deemed as a product change to refine how the People picker control validates as the text is entered into the control rather than when the Nintex Form is submitted.

The link for this request is here

Please vote for this so that our Product Management team are aware of the demand for this change.