How can I avoid duplicates between Leads and Contacts? For instance, when I send an email to a contact's secondary email address, a new lead gets created instead of linking the email to that contact.

How can I avoid duplicates between Leads and Contacts? For instance, when I send an email to a contact's secondary email address, a new lead gets created instead of linking the email to that contact.

You can enable the duplicate check by using the email address as a unique field. Follow these steps to enable the duplicate check:
  • Go to Setup > Customization > Modules and Fields. 
  • Select the Leads module.
  • Hover your mouse over the Email field.
  • Click on Settings > Do not allow duplicate values.
  •  Click Save Layout.