Photos Library: .photoslibrary file taking double of storage space in Photos app and Document Folder
The reality is that there isn't two files taking up the double storage. The storage is only used once.

The view you're looking at is just confusing, since the "Fotos" and "Fotos-Mediathek" is really one and same.

You can open your home folder in finder and select the list view (command 2) and then calculate all folder sizes. Let finder add up all the space and you can verify you don’t have double space used.