Here is what I am doing. It may simply be that I'm confusing the actions.
1. Open "Photos" app.
2. Select "Photos" at the bottom. (There is also "Shared" and "Albums".)
3. The window title is "Years" and I select "2016".
4. The window title is "Collections" and I select a location.
5. The window title is "Moments" and I select a photo to edit.
6. I click "Edit", crop the photo , click "Done".
7. The edited photo is displayed and I click the share icon (the square with the arrow pointing up).
8. The apps that are available include "Message", "Mail", "Google Drive", "Gmail", etc. The actions that are available include "Copy", "Duplicate", "Slideshow", "AirPlay", etc. There is no "Camera Roll" or "Share duplicate" option anywhere.
9. I select "Message" and the original photo shows up in the draft message.
When I tried the "Duplicate" option I selected both photos and both photos showed up in the message as the original.
Thank you very much for your help.