Frustrated. Trying to import a custom field to all accounts - creates duplicate accounts

Frustrated. Trying to import a custom field to all accounts - creates duplicate accounts

I have a custom field that I wish to upload for all of my accounts. 

I have exported my list of accounts to get the Account ID
I then import the CSV with the new custom field column.

When the data is imported duplicate accounts are created. I have correctly mapped the fields including the Account name and Account ID so not sure why duplicates are created.

Please help, very frustrating.