How to check if an item exists in a list

Hi All,

I have a requirement where users need to fill a form and selects a period such as 1st Half FY2017, 2nd Half FY2017 and I have defined period column as choice field as shown:

The requirement is that users can should not the same period for e.g. if a user has already filled in 1st Half FY2017 in the current list and some other user fills in the same period i.e. 1st Half FY2017, the form show should show user that this period “1st Half FY2017” already exists in the list.
 

How could I achieve this?
I have come across a question about the same https://community.nintex.com/thread/1584  Check if item is unique before adding list item / Delete duplicate items but its using Nintex workflow to check item exists using Query list.

ddd

• What Query List configuration is required to do this?
• Is it possible to do the checking within the Nintex form itself? Javascript? 

Any suggestion or pointers would be highly appreciated.

Thanks in advance.