This article covers
How to merge customer records with the same email address?
How to match several subscriptions that belong to one customer?
To avoid creating duplicate customer records, that applies when an existing customer checkout using hosted pages, you can use the API functionality as stated below.
Capture and save the email address of your customer on a custom-built form in your website
Use the List customer API to check whether an existing customer record with the same email address is present or not.
If a customer is present with an existing email address
If an existing customer is not present, use the create new subscription or checkout new hosted page API.
if you have handled duplicate subscriptions, and if you need to manage the existing duplicate subscriptions use the merge customer API.
Related articles and Documentation