If you already have a client listed in your CRM, you can link the existing client to any Opportunity. This prevents you from having to enter client details twice.
From the Opportunity >> Contacts section, click on the link icon
Search for the client's name, highlight, and click choose
You can also link Assets and Liabilities from the A and L section
You can also move (not copy) existing Calculators to a new Opportunity from the Calculations section