Universal Containers needs to allow customers to self-register for their Community for customers.

Customise the CommunitiesSelfregcontroller Apex controller to identify duplicates and show an error message during registration.

Uncheck the option Allow Duplicates in Salesforce in Community Management.

Create a validation rule to prevent duplicates and show an error message.

Develop a batch process that will delete duplicate contacts in the Community and reassign users to existing contacts.