When to merge data in a form?

I’m wondering how to make UX better in such a case

There is CRM, as a user of this CRM, I can create a customer with fields phone, email, name, address. Sometimes it’s possible when a customer with this contact information has already existed. So, as a user of CRM, I need to merge the data.

When is it the best time to make it? As soon as email or phone number is filled in and we found a coincidence? Or as a user clicks “create”?

In the first solution, I should change the title after merging “Update info” or so on, because it’s not creating even more. And it can confuse a user. The second solution supposes to fill in an entire form and after a creating to see “merge popup”.