[Ortep]

I have a paperwhite 2 and I was playing around with it. Sideloading books, creating collections etc. After a couple of days I found that several books showed up 2-3 times in my library. These were from old experiments. I could not delete them. Checking with a USB cable showed that thet were actually not on the Kindle. Restarting did not help. So I decided to start fresh with a factory reset. That went fine an everything was gone. I sideloaded a series of books and I tried to create the collections again. But the Kindle won't let me. It tells me the collection name already exists. But it does not show up on the list. So I cannot add books. How doe I get rid of these hidden collections?