You see the message as "Duplicate key value violates unique constraint “unq_app_contact" only if you have added the same email ID to multiple V4S Kiosk users. We suggest not to enter the same email ID to multiple contacts in Salesforce.

Each V4S Kiosk User must have a unique Email ID to login to the V4S Kiosk application. The V4S Kiosk application searches for the email ID and the related Contact in Salesforce when an ID is entered in the V4S kiosk login screen.

If there are multiple Contacts in Salesforce with the same Email ID, then the app does not know to which contact is the Email ID linked to, and the login fails with the message as "duplicate key value violates the unique constraint, unique_app_contact"

We assume you are adding the same Email-ID to multiple contacts because the Volunteers are from the same family. Instead of adding the same Email ID to multiple Contacts, please use a fake ID for family members/Contacts.
e.g.: If there are Volunteers from the same family as Father and Son. Please add a proper email ID to the main contact (Father) and add a fake email ID to family members (Son).
Please note that if you add fake Email IDs to Minors, they cannot reset the password using the "Forgot Password" option on the V4S Personal login screen. They can only reset the password within the V4S Personal app using the change password option from the V4S Personal Settings screen.
We suggest you inform minors when they login to the V4S Personal app to enter a password that they can easily remember for their future logins.