I can't tell you why you have duplicates because it will depend on the detail of how you transferred from windows.
First - is your mac synced with icloud photos. If so, your deletions in iCloud will soon be synced to your mac, and they will be deleted on the mac as well. If you are synced with iCloud you need to undo the delete now. (Restore from recently deleted folder).
Then disconnect the mac from iCloud before deleting them on iCloud again.
Regading getting rid of the duplicates:
There are no facilities in photos to automate this, so either
* manually,
* or using an app designed to work with the photos library (Ie one that marks the photos or puts them in an album, and lets you review and then delete them, rather than doing the delete itself.
Do NOT use a general file duplicate finder not designed to work with the photos library. They will damage your photos library and likely cause loss of your photos.
I personally use PowerPhotos (I also use this for copying images and data between libraries)
https://www.fatcatsoftware.com/powerphotos/
Another suitable app recommended by experts here is PhotoSweeper (I have no personal experience of this one)
https://overmacs.com/
Both are paid apps.
