I would like to delete all files of my owned movies
You can un-associate the files with the menu "Tools->Manage my video files->Batch delete several file associations".
For example, entering the string "c:" will remove all the files from your C: drive.
or delete the entire database and start from scratch.
To start from scratch, you have to delete your coollection file.
From what I have understood in this forum I have located the .cool file and deleted it together with all the backups after closing the program.
However, upon re-opening I found all my owned movies still listed.
Visibly you haven't deleted the right file. You can use the menu "Help->Locate my coollection file" to find the right location.
More details about coollection files:https://www.coollector.com/help.html#coollection
I would also like to know how to delete duplicate files. After having done my scan I see duplicate files listed but how to delete them and how to choose the one you want to keep?
If the files have been moved and don't exist on your disk anymore, you can use the menu "Tools->Manage my video files->Find associations with missing files".
If you have several copies of the same movie and want to clean up your disk by deleting some duplicates, you can use the menu "Tools->Manage my video files->Find duplicate video files".