One simple way is to create a Public List. I got this modification from Martin.Ostlie. Tony Visconti has created a stored procedure but if you haven't done that before it takes more learning to do.
To create a Public List to find duplicates:
1. start a new list and Name it and categorize it, Next
2. don't select any criteria on the Criteria page, Next
3. Select these columns in this order: Person ID, Last Name, First Name, Suffix, Street Address, City, State, Zip Code, Email
4. Sort Last Name, First Name
On the final page open the Query button and delete the whole query. Copy the query below and paste it in place of the old query. Finish.
Copy and Paste this query:
SELECT
person_id,
last_name,
first_name,
suffix as person_suffix,
dbo.fn_Age(birth_date, GETDATE()) AS age,
street_address_1 as address,
city,
state,
postal_code,
email
FROM core_v_person_basic AS a
WHERE person_id NOT IN (SELECT DISTINCT person_id FROM core_profile_member WHERE profile_id = 3865)
AND 1 < (SELECT Count(*)
FROM core_v_person_basic AS b
WHERE b.first_name = a.first_name
AND b.last_name = a.last_name
AND b.first_name <> '')
ORDER by last_name, first_name ASC