The config, "Stop Duplicate Email and Phone Number" is used to not allow a duplicate phone number to be used.
If this config is on, the system will give an error message in case a customer uses the email or phone number at the time of enrollment which is already added to another customer's account.
Steps to access this config:
Click on Settings Icon>> System Config (PC234)>> Enrollment section>> Stop Duplicate Email and Phone Number
Here's the link for the video showing how this config works -