How to disable the unique question validation?

when my customers try to submit more that one registration form from the same device they are getting an error message that they can’t submit more than one form.

That happens when you have limited submissions on your form, you can check the following documents about it in order to know what the feature does and how to disable it: 

https://www.jotform.com/help/64-How-to-Set-Unique-Submissions-on-a-Form

https://www.jotform.com/help/223-How-to-set-Form-Limits-Based-on-a-Unique-Question