Marketing Cloud treats leads and contacts as separate records, even if they have the same email address. This means that syncing leads from Sales Cloud to Marketing Cloud can create duplicate contacts and increase the contact count and billing. To avoid this, the marketing team should consider using contacts only or converting leads to contacts before syncing them to Marketing Cloud. Reference: https://help.salesforce.com/s/articleView?id=sf.mc_co_sync_leads_and_contacts.htm&type=5