Document how cbimport handles duplicate documents

The customer put a question regarding the cbimport tool -   If they run cbimport to insert seed data into our database, but the seed data is already there (perhaps in an old format, and the data to be inserted is the "updated" and correct version of the data), what will the tool do?  Overwrite?  Fail? 

Need more explanation/clarification in our documentation on how cbimport deals when loading data which is already in the CB server.