how to show alert or user message on public facing web site as like SharePoint for remove duplicate detection on Contact US form.

Dear Support,

I had successfully added Contact Us from app on SharePoint 2013 online public facing web site and inserting data in SharePoint list as a back end.

I had added custom column in SharePoint list and selecting Enforce unique values “Yes” for remove duplicate detection as per attached screenshot 1.

But I am getting issue when I am inserting duplicate value in custom column by public facing web site its does  not showing User message or alert message on public facing web site just like email id is already exist when I am inserting duplicate email id its showing “something went wrong “ as per attached screenshot 2.

Please let me know how to show alert or user message on public facing web site as like SharePoint for remove duplicate detection on Contact US form.

Referred URL:

https://support.office.com/en-US/Article/Add-a-Contact-Us-Form-app-to-your-website-4d6133f7-2aaa-4cb0-9e0d-bebbfefd00e6?ui=en-US&rs=en-US&ad=US

Would request for your support.