Marketo has a system smart list that finds duplicate people by matching their email addresses. If you want to use another field to find duplicates with, here’s how.
Select your smart list, click on the Smart List tab.
Find and drag the Duplicate Fields filter onto the canvas.
Choose one of four available options:
All fields, with the exception of Email Address, are case-sensitive. So using “john doe” in the Full Name field would not return results for John Doe.
Done! Run the smart list to find people with the same value in the previously selected field.