Eek! My Photos app says it's going to delete a huge bunch of my images in 28 days! I have a MacBook Air and just updated to macOS Sierra, which automatically installed the "Photos" app to replace my old "iPhotos" app. As I scrolled through the "Photos" app sidebar I checked to see that all of my photos were there in the "Photos" folder (at the top of the sidebar). Then I scrolled down to the "Recently deleted" folder and to my horror saw a huge number of my images shown under the advisory that these photos would be permanently deleted in 28 days! They are  photos that I didn't delete from iPhotos or Photos, and I don't want them deleted.
I looked again at the "Photos" folder and saw that those same photos — the ones destined for deletion in 28 days — were also showing up in the "Photos" folders. I don't need two copies of them but I am wondering if they are safe if I see them in the "Photos" folder?

Now I did keep copies of my images both in my old iPhotos app and in another photo app (called My Pictures), and I did recently delete a large number of them from my "My Pictures" file to get more disk space, but I didn't delete the duplicates of those same photos from my iPhoto files. Is it possible that the photos I now see as listed for deletion (under Recently Deleted) are those duplicate photos I deleted from my My Pictures folder?
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".