Importing Contacts - to update a field

Importing Contacts - to update a field

I am trying to update a field that was not originally imported into our contacts.  I selected the overwrite button when I imported a test section - but it cloned the contact rather than overwriting it - so I ended up with two contacts one with the new field filled out and the other without it.  I have about 6000 records that I need to update.  Thank you!