Duplicate photo libraries
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.