Shopify Plus has a feature called Multipass -- https://shopify.dev/docs/admin-api/rest/reference/plus/multipass
Multipass login is for store owners who have a separate website and a Shopify store. It redirects users from the website to the Shopify store and seamlessly logs them in with the same email address they used to sign up for the original website. If no account with that email address exists yet, one is created. There is no need to synchronize any customer databases.
Consider the scenario that a user performs two multipass logins where the user's last name is passed as an optional field, but prior to the second login attempt, the user changes their last name. Does the second multipass login, which passes the new last name, result in an update to the Shopify customer record?
Further down the same page, it states that
Shopify uses email addresses as unique identifiers for customers of a shop. When registering customers in Shopify, the merchant must set the unique identifier in the "identifier" field in the following cases:
The site uses other identifiers (such as usernames) Two different users of the site might be registered with the same email address If the email address is always unique, setting the "identifier" field isn't required.
Only one Shopify account can use a specific email address. Registering a second customer with the same email address (even with a different "identifier") will result in an error.
This means that user is identified using unique identifier, that may be email or something else if specified. So in above scenario, if the email is same and only last_name or any other attribute is changed, a new user will not be created but the information will be applied to existing user identified using that email.