Duplicates

I am in process of creating a report with the below details.

person_id,membership_id,first_name,surname,create_datetime,first_contact_date,promotion_id,promotion_description,description,heard_about_text

I am getting duplicate values for person_id with null membership_id and I am not sure why?

 SELECT 
      DISTINCT dbo.pn.person_id,
      dbo.pn_membership.membership_id, 
      dbo.pn.first_name,
      dbo.pn.surname, 
      dbo.pn.create_datetime, 
      dbo.pn.first_contact_date,
      dbo.promo_reference.promotion_id,
      CASE 
        WHEN dbo.promo.description is NULL THEN 'Promotion Not Set'
        ELSE dbo.promo.description
      END AS promotion_description,
      dbo.promo_sales_channel.description,
      dbo.memship_app_eligibility.heard_about_text
   FROM  
      dbo.pn
      LEFT OUTER JOIN
      dbo.pn_membership 
      ON dbo.pn.person_id=dbo.pn_membership.person_id
      LEFT OUTER JOIN
      dbo.promo_reference 
      ON dbo.promo_reference.main_ref=dbo.pn.person_id
      LEFT OUTER JOIN
      dbo.promo
      ON dbo.promo.promotion_id=dbo.promo_reference.promotion_id
      LEFT OUTER JOIN
      dbo.promo_sales_channel
      ON dbo.promo_reference.sales_channel_id=dbo.promo_sales_channel.sales_channel_id
      LEFT OUTER JOIN
      dbo.memship_app_eligibility
      ON dbo.pn_membership.membership_id=dbo.memship_app_eligibility.membership_id
      WHERE dbo.pn.create_datetime >= DATEADD(Day, DATEDIFF(Day, 0, getdate()), -1)
      AND dbo.pn.create_datetime < DATEADD(Day, DATEDIFF(Day, 0, getdate()), 0)
      ORDER BY dbo.pn_membership.membership_id,dbo.pn.create_datetime desc

Sample Data:

25 NULL Pat 2014-01-14 18:06:53.407 NULL 3 No Promotion Paper NULL
25 NULL Pat 2014-01-14 18:06:53.407 NULL 3 No Promotion Phone NULL
26 525 Eri     2014-01-14 15:00:00.412 NULL    3       Promotion       Phone   Internet