[Coroner]

Yes, you have to register.

There is no "duplicate finder" - cause all files are unique. That means, you can upload the same file more than 1 time and all will have a unique filename.

You can delete "media by media" or if you in a category, mark the checkboxes and delete them all.
Media inside a category (viewed page) could be: 3,6,9,12,15,18,21,24 by default. But you can enter the steps inside the AdminCP (columns inside the category). F.e. if you choose 4, the steps would be: 4,8,12,16,20,24,28

This means, you can delete X files at the same time.