Eek! My Photos app says it's going to delete a huge bunch of my images in 28 days!
When you "delete" an image in Photos, what really happens is that the photo gets moved to the Recently Deleted folder, and only after 30 days is it then actually deleted. Photos in the Recently Deleted folder do not show up in other areas of the Photos app, so if the only images there are also showing up in the "Photos" section, they must be duplicates that you deleted.

If you want to delete photos now, you can either click "Delete All" in the top right corner, or select the photos you want to delete and the button will change to "Delete" and only remove those that you selected.

If there are photos there that are not duplicates that you want to delete, you can select those and click "Recover" in the top right, and they will be moved to "Photos".