The import protocol of a database keeps trach of all the databases that were copyied into it. ChessBase uses this to warn you if you try to copy the games of a database twice.
You can view the import protocol by clicking Show protocol in the databases properties (right-click a database icon, select Properties – Show protocol).
Clicking Reset protocol will erase the records.
The information about which games have been imported is stored in the “INI” file of the database.