Photos Library

If your photos are still in iCloud Photos, and the library is looking well on your iPad and in iCloud, you can recreate the Photos Library on your Mac by downloading a fresh copy from iCloud.

 I suggest to try the following:

• On your Mac launch Photos, while holding down the options key ⌥.
• Keep holding down the ⌥ key until the dialog to select a library appears. Then select to create a new library.
• When the empty, new library opens in Photos, open the Photos > Settings > General and click on "Use as System Photos Library".
• Then click the iCloud tab in Settings and enable "iCloud Photos"

Now wait for your photos to download from iCloud to your mac. This should work, if your Photos are already in iCloud. The only thing you cannot get back from iCloud Photos are your projects (Photo books, Slideshows), as they are not syncing with iCloud.

Once Photos is working again on your new Photos Library, you can try to to clean up the mess in the Pictures folder. But do not use duplicate finders on your Photos Library, unless you are sure, it is a safe app, that does not damage a Photos Library. Some Duplicate Finders are dangerous and can damage your Photos Library. This can cause Photos to act strangely or unreliably.

Photos can open only one library at a time, but it should not delete a Photos Library, when you create a new library. If it looks like you lost the older library, Photos may not be able to access it, if the library is in an unsupported location (in a shared folder, on a NAS, on a volume used for Time Machine backups, in a Drop Box, on a volume with an unsupported file system format). In that case it might help to move your old library to a supported location, where Photos can use it safely (Move your Photos library to save space on your Mac - Apple Support)