I agree that keeping a record of the files that you've processed is best way to accommodate your requirements, but it doesn't have to as complex as the previous suggestion. You could simply pick up each file, check the file name against your ProcessedFiles (for example) table; if there is no matching entry, continue to process the file; if there is a matching entry, do nothing further and move on to the next file.

You can also record other potentially useful information in your ProcessedFiles table such as row counts and timing information. It is also useful as a basic audit table.

Regards

Lempster