The duplicate checking depends partly on the data that you are mapping in.
In most cases Bloomerang Account Number works as a perfect match for importing, unless there are also duplicate addresses in your file. In this case, you should also include First and Last Name to make sure all the duplicates match up.
If you do not have the Bloomerang account number, it will also find duplicates based on the following combinations of data:
First and Last Name + Address
First and Last Name + Phone
First and Last Name + Email Address
If you have both Organziations and Individuals in your file it is recommended that you separate them into different files otherwise the address, phone and email address duplicate checking will not work correctly.
First Name matches as an includes or tokenized match and allows "Micah & Liz Weaver", "Micah Weaver", "Liz Weaver", and "Micah and Liz Weaver" to match a joint account in the database named "Micah & Liz Weaver" or "Micah and Liz Weaver"