Trying to Import Overwrite Contacts - it's duplicating them

Trying to Import Overwrite Contacts - it's duplicating them

I am trying to add a Nickname field (that I failed to map in a Migration) to existing Contacts. I ordered a backup, took the Contacts1.csv, removed unnecessary columns, added the Nickname column so that it looks like this:

Contact ID Last Name Nickname
zcrm_1321927000000238739 Rosenbaum Rich
zcrm_1321927000000262920 Kahlbaugh Rick

I then tried to Import it from the Contacts Page (not via Migrate), using Import/Overwrite, and mapping the Last Name column. The result is always 2 new Contacts - it doesn't match on the Contact ID, as the help file suggests. And I find no place to map the Contact ID field, so I assume the import process recognizes it without my map? And I've looked up those 2 contacts and the Contact ID in the URL matches the Contact ID in the file, less the "zcrm_", of course. Can you help me please?