Q: Are pictures saved twice on my MacBook?

The free App "FreeDiskSpace" avaiable in the Mac-Appstore tells me, that on my HD there are 30GB occupied by the iPhoto-library and 28GB by the Photos-library each. Can I delete the 30GB-iPhoto-Library without losing the pictures in "Photos"?

It only looks like the iPhoto Library and the Photos Library had doubled the storage used by photos.  But Photos and iPhoto are sharing the image files.  Photos uses hard linked copies of your original files in iPhoto.  See this document:  Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support

You could delete your iPhoto Library, but keep a backup copy. Deleting the iPhoto Library will not free much storage.  And you may need the original iPhoto Library  to fix problems, if you notice that the Photos library did not migrate correctly, photos are missing etc.

No your pictures are not stored twice - they are stored once and hard linked to both libraries - Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support

Yes you can trash the iPhoto library but it will not save much space and is not a good idea since you save little and you may want or need it in the future and getting it back can be difficult

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