You can easily avoid sourcing from existing contacts or companies that are clients or opportunities by simply connecting Salesforce via oauth and following the following easy steps.
Step 1: Go to your account settings page, found in the top navigation bar under "Account".
Step 2: Go to the Suppression Lists section, click on the "Add Companies" dropdown and select "Setup Integration".
Step 3: The setup integration wizard will appear, from which you can select your CRM by clicking on the according setup button. This will initiate a redirect to your CRM to ask for data access permissions. These are needed so that we can identify all companies, contacts and leads you want to suppress.
Step 4: You will be redirected to the suppression list configuration wizard, from where you can select the opportunity stages that you want to suppress. Select all current clients, opportunities but avoid selecting leads or subscribers, otherwise too many companies will be excluded.
Once you continue to configure your "Data Export" or if you simply close the wizard, all current companies with the selected stages, contacts or leads in Salesforce will automatically be suppressed. This may take a couple of minutes, depending on your CRMs size. Any changes in your CRM will automatically be taken into account.
Congratulations, you no longer need to manually de-duplicate your newly sourced contacts and companies!