Duplicates in your CRM can happen as a result of human error (process issue) and non-human error (systematic issue). Here's how to address each:
Process Issue: Sometimes a salesperson adds a new lead in your CRM for a customer that already exists. When manually adding leads to your CRM, it is a best practice to search by customer contact information (email, phone) to see if the customer record already exists before adding that as a new lead.
Systemic Issue: Each CRM is unique when it comes to handling duplicates. Some do a better job of de-duplicating incoming leads. Contact your CRM provider to discuss how they can help you prevent duplicates.