If you repeatedly run inventory into the same database, the results will be additive:
- Any machines already successfully inventoried into the current database will not be re-inventoried.
- Any machines not successfully inventoried previously (such as access denied or timed out.) will be re-attempted for inventory.
- Any new machines discovered (such as machines that were not previously in Active Directory but that are now there, or new machines imported via flat file) will be inventoried.
Cut and paste from: Microsoft Assessment and Planning Toolkit Frequently Asked Questions (FAQ)