TextEdit saves changes to file without prompting when you duplicate

I opened a file and modified the contents, then selected duplicate meaning to save the duplicate as a new file. I closed the original, expecting rekect the prompt to save changes... instead it closed the file with my changes without any prompting at all. My original file is apparently just gone.

Unless I have found a bug (unlikely) I must be misunderstanding how duplicate functions. In lieu of save-as (from Windows world) what is the correct way to achieve the same thing without risking loss of data?

macOS has for some time now, by default, automatically save changes to a document in TextEdit without prompting. Make your duplicate first, close the original, then edit the duplicate.

If you want to be prompted, then go to System Preferences > General and check the [√]Ask to keep changes when closing documents checkbox.

This works for most all Apple application's and some third-party application's. Some will prompt either way.