When you import records, the Import Wizard includes the functionality to check for duplicate records. When enabled, Sugar will check to see if a record already exists in Sugar with the given criteria. Sugar will then mark any record as a duplicate if it cross-references against any of the fields in the Fields to Check column. You also have the option to choose whether the fields to check are combined using an AND or OR operator so that you can find the duplicates important to you.
For example, when importing Contacts, if you select First Name, Last Name, and Email Address as your fields to check and the operator is set to OR, any record that has a matching first name, last name, or email address to an existing contact in Sugar will be marked as a duplicate.