How can i make sure that the client entering the data did not reuse the numbers

https://form.jotform.us/LCCAP/risk-inventory

Please rank the 8 indicators of risk 1-8 with 1 being the most significant indicator of risk and 8 being the lowest.

        Personal Injury (drop down box next to each to indicate number?)

        Property Damage

        Employee Practices

        Legal

        Agency Reputation

        Financial

        Operational

        Political

How can i make sure that they would not use the same number twice ? 

The easiest way to do this would be to use an orderable list widget:

http://widgets.jotform.com/widget/orderable_list

(manual: How to Add a Widget to your Form)

This will allow for creating a list that users can reorder in the form.