can i copy my photo library (original) to a folder on my desktop

Photos is creating working copies of your photos, for example thumbnails of the recognised faces.

But there are no duplicates of the photos, even if photos is showing you the same photo in many albums. Each photo will be stored only once, but shown in many albums for easier access to your photos. You are seeing the photos organised by the people in the photo, by the locations, by the capture time, by the media kind. But there are no duplicates. If you delete a photo from any album, it will be deleted from all other albums as well.

Where is your Photos Library now?

You can copy the library to any local folder on your mac in your user home folder, also to the Desktop.

The default location is the Pictures folder, however. Photos is looking there for the Photos Library by default. If you move the library somewhere else you may need to tell Photos where to find it.

Are you using iCloud Drive? You must not put your Photos Library into any folder that is syncing with iCloud Drive. If your Desktop or Documents folder should be syncing with iCloud, you must not keep a Photos Library there. It will be damaged by the syncing. The same goes for other cloud storage, like DropBox or Google Drive.