At first, you can change the note type to Basic (and reversed card) so you will have both cards front-back and back-front (English-French and French-English).
If you really need to duplicate the deck try this:
- Export your deck
- Make a second user in Anki
- Import the deck
- Rename the deck
- Change the card template so French is on front an English is on back
- Export it, switch the user and import the new deck.