The easiest way to perform this would be to set up a duplicate finder task and run a mail merge on the populated group.

To set up the Duplicate Finder task:
  1. Go to Data Management > Tasks.
  2. Select the Task List tab.
  3. Select create a new task.
  4. Select Duplicate Finder.
  5. Enter a name and description of the task. Note that the automated group will reflect the same name as the task.
  6. Mark the radio buttons for the appropriate parameters that the system should consider when determining if the record is a duplicate.
  7. Save your changes.
Once you run the task, a group by the same name of the task can be found under Constituent 360 > Groups. Once the task has completed, perform a mail merge on the group.