How to implement lead nurture in Marketing Cloud & avoid duplicates?

Question

We are migrating from Salesforce Pardot to Marketing Cloud . Currently in Pardot they have lead nurture form where a user sign ups in form and when they qualify , signed up user get converted as lead .

I do not want to create a lead on smart capture form submission (where i can check if lead exists or not using RetrieveSalesforceObjects & CreateSalesforceObject ) instead I want to create Lead only if they nurture & Qualify as a lead . Because not all records who signs up the form are leads since some are scam as well.

How to implement this scenario in Marketing cloud ?

We are re-building the Pardot forms(newsletter & other campaign forms) in Marketing Cloud cloud pages & when customer signs up in the form , send them an email . When the customer qualifies as lead based on some criteria, create a lead record and sync it up in salesforce ?

So I have created a smart capture form and sending email to them .

The problem is occurring when I try to create a lead record if it gets qualified

For the same email address 2 records are getting added to contacts .

1. When user A ([email protected]) signs up via smart form and email needs to sent, so record is getting added to all subscribers with subscriber key
2. When the user A ([email protected]) is qualified & created as a lead in salesforce, lead record is getting synced into Marketing Cloud with same email address resulting in duplicates.

So for same email address 2 records are added into marketing cloud

| SubscriberKey | EmailAddress | | [email protected] | [email protected]| | 005F0000NXAAa2| [email protected]|

How can I overcome this issue ?