Thanks to the Answer of Martin H. Bramwell and the comment of Ray Carnes I tested different things and found out what to do. I hope you both don't feel I steel your answer. I only try to explain what to do in small steps and what happens in my own words.

The method to get rid of __export__.product_product_19_26 and that kind of things in the export:

1. Inspect your export.
2. If you have this kind of rubbish in your export check the column name.
3. Often this will be category id or oum id or others or both or more.

For each of these you or your user has made new categories, uom, or other. For reasons I do not understand the database is not ready for export and produces rubbisch in your export.

1. Make an export of the above found categories, uom or other. If you do for some reason I do not understand the database is made ready for usefull export.

2. Export again and be happy!

In my tested case the rubbisch has been changed in logical data:

• Data in uom_id/id was changed in __export__.product_uom_10 /11/12 etc which was not unique and corresponding with the data in the other exported records.
• Data in uom_po_id/id has been changed in __export__.product_uom_10 /11/12 .
• Data in categ_id/id has been changed in __export__.product_category_19 /etc..

This is great and for me this is a breaktrough in the problem of export / import!