Because a support desk, a billing system and a CRM each create their own copy. The same person then has three email preferences, two job titles and one very confused experience of your company.
A contact is one person: name, role, contact details, the company they belong to, what they have consented to, and every interaction recorded against them.
There is one contact row, referenced by every product rather than copied into it. A change of role or email is made once and is immediately true everywhere it is read.
A procurement lead at Kanaka Foods moves to Bexley Group. Reassigning the contact keeps their history against Kanaka Foods where it belongs, and starts a clean relationship at Bexley with the same person recognised.
It is not a personnel record. Employees live in Nest, and a person who is both a customer contact and an employee is deliberately two records with two different purposes.
Desk reads it to see who is raising a ticket, Books to address an invoice, Loop to name a client approver, and Growth to attribute a form fill to a known person.
One primary company, with a recorded history of previous ones. Dual current roles are held as a relationship, not by duplicating the person.
Whoever has visibility of the records that history sits on. Permission is per record, not per product.
Mark them inactive. History stays, and sequences and marketing stop targeting them.
Fourteen days, every module, no card. Or half an hour with someone who will run it on your own records and tell you where it does not help.