I have a question about duplicate photo libraries that seems the opposite of a previously asked question. In my case, after converting iPhoto library to Photos library, the computer at one point asked if I wanted to convert Photos library to iPhoto library. So I said yes and now have two libraries. In this case I think each one is taking up 130 GB of space rather than each library pointing to the same photos, and I would like to remove one of them. Is it safe to remove the iPhoto library? Would this also delete the photos used by the Photos library?
If you make a time machine backup, you can be sure that you can recover from anyone's guess as to whether you have two libraries or just that the files are linked together.
Photos is designed to use hard links in some cases to avoid duplicating every photo during the transition period. When you eventually delete the old library, then you no longer have linked files. Keep in mind - when files are hard linked, you have two pointers to the same one file. No duplication happens other than at the level you are accustomed to seeing things in Finder and open/save dialogs.