I'm importing a CSV file with about 2500 contacts from another system, outside of SalesForce. The wizard is matching based on name. So if I import the same spreadsheet again, the data doesn't update, but instead it creates duplicates. The data I am importing has a unique User ID field that is 8 characters long. But after reading for a while online, I found out that External ID field cannot be used for the default Contacts. What is the best way to prevent duplicates when importing bulk data? The imports would happen once a month and this is for non-profit organization, so it has to be free if 3rd party plugins will be needed. Thanks for any ideas!