How to avoid CRM to check closed lead's email address when Import and update existing leads

Hello,

I am trying to update existing leads by using custom entity which is called Lead Update and workflow. I am exactly following the following link

http://www.powerobjects.com/2013/08/01/updating-records-in-microsoft-dynamics-crm/

When I imported the records into Lead Updates, I got some errors like "A duplicate lookup reference was found",  I checked up Leads entity and found we have some closed leads with same email address as some open leads. My question is how can I limit the CRM just check the open lead's email address when do the lookup?

Thanks  a lot!

Carys