Hi,
I had the same problem and managed to find this https://developer.sugarcrm.com/2011/11/08/whats-new-in-sugar-6-3-import-duplicate-fields-checking/
In summary you need to create an index for the field that you want, that means going to the database and executing SQL command -
ALTER TABLE accounts_cstm ADD INDEX newTextIndex(newText)
change ‘account_cstm’ to table and ‘newText’ to the field you want.
Good Luck!
Jason