QUESTION
Why is it not possible to import the files I deleted from the Catalog before?
ANSWER
When you delete an image from the Catalog, but not from the disk, it goes to the Catalog Trash. Therefore, if you want to re-import that image with the Exclude Duplicates option, Capture One will skip it during the import.
Instead, you can go to the Trash folder in the Library and drag the image into a different folder in the Catalog.