This is why it is important to save as psd or tiff first for the master file. When saving as a jpg you don't see the layers being flattened in photoshop, it is done behind the scenes. But you you fail to save as a psd or tiff and close the document, then it is too late, as opening the jpg image would show a flattened image.
I think I may have tried Image>duplicate once since I started with photoshop way back in CS2, but I just got in the habit of making sure, I had a master file first, just in case. Since saving a jpg does not flatten the layers in the current document, you can save as psd or tiff next as long as it is done, before closing the document.
However, if someone was to manually merge or flatten the layers, then it could be too late to save the master file, if that person went beyond the maximum undo's allowed for the history panel. This would be one time where you would want to save the master file first or use Image>Duplicate. And this is why I make it a habit to save first then merge or flatten.