Duplicate primary key in bulk insert after truncate in SQL Server 2008?

Question

I'm using SSIS to dump data from my SQL Server 2008 prod database to flat files and then using bulk insert to load those flat files into the SQL Server 2008 dev database. The SSIS package is running a TRUNCATE TABLE statement before the bulk insert, but when I run it I still get this error: "The statement has been terminated. Violation of PRIMARY KEY constraint. Cannot insert duplicate key." Why would there be a duplicate key after a truncate? Is there something more I need to do to clear the primary keys before the bulk insert?