So far I solved( a lot)

I uoload a excelfile with a form to a specific folder --> works
The I run the import cron -- Works
but:.
everytime I upload the file again, run the import cron, it does not override( should!) so I have double articles.
My customer has a excllist with products.
This products will change, some new ones, some no more availlable. (This I solved with state = 0 --> works)
But how can I avoid double entries?!
Please give me a hint:-)
Thanks Klaus