Photos App - You are opening the Library for the first time

Marko,

are you seeing the message, when you double click the library in your pictures folder to open it?

You may indeed have been using a different library, if Photos is warning you that you are opening the library for the first time. To see, where the library is, that you have been using recently, launch Photos by double-clicking the Photos.app in the Dock or in Applications. After Photos launches, open the Preferences: "Photos > Preferences > General". Look at the first line of the Preferences panel. It will tell you the path to the Photos Library. You can also click the button "Show n Finder". Does that show you the library in your Pictures folder or another library?

If Photos is really opening the library for the first time and you are using iCloud Photos, the library will be uploaded again to merge it into the existing library. That will always happen, after the library has been repaired or the library has been restored from a backup or moved to a different volume. It will not create duplicates in iCloud, just cause a lot of network traffic and take a long time.

Try to restart your Mac in Safe Mode, then back to normal. Safe Mode will clear some caches. Then try ry the repair again. This helped in some cases to get the Repair done.

See: How to use safe mode on your Mac - Apple Support

If this still does not fix it, create a new, empty library to check if Photos is working at all, at least, if you are not using your main library. Creating a new library is explained here: Create additional photo libraries in Photos on Mac - Apple Support

Sometimes switching to a new library helps to get preference files reset, so Photos can remember the library location.

Photos has only the one repair tool. If it cannot fix the library, you will have to restore the library from your backup.