Gallery is showing photos that are on your phone - internally or on your SD card. When you delete a photo, whether you use Gallery or a file manager to delete it, you're still deleting the same file. (Gallery isn't a place to store photos, it's just an app that shows the pones on the phone.) If you don't want a photo showing in any app, add a file to the folder the photos are in, named .nomedia (with the dot before the n). That keeps media apps (like Gallery) from showing them, but the files are still there. But moving them from what you think is Gallery to the SD card doesn't give you 2 copies - Gallery is just showing the photo on the SD card.