Sales Cloud: Duplicate Value Found Error During Lead Conversion Due to Unique Field Mapping

This error is the result of any unique field on Leads being mapped to a unique field on Contacts, Opportunities, or Accounts, where the unique field has the same value as a unique field on a record that already exists.  

For Example:

1. There is a custom field on Leads and the Unique checkbox is checked.
2. There is a custom field on Accounts and the Unique checkbox is checked.
3. Lead Field is mapped to the Account.
4. There is an existing account with the same unique value already populated.
5. Upon lead conversion, the mapped value will cause a "duplicate value found"  Error  when the New Account is being saved on create.

* In Classic UI, this error is displayed below the Converted Status Field on the Conversion page. In Lightning UI, it is displayed on the top of the Conversion modal dialog.

To fix this issue, delete or edit the unique value on the Lead so that it does not violate the uniqueness constraint. This allows for a successful conversion.