duro means difficult, but also harsh, tough, robust

"Duro" has a broader meaning: it is literally related to the consistency (hard), but in a metaphorical way indicates something difficult as well (difficult) On the contrary, you'd use "difficile" just to describe something difficult and/or demanding. For istance: - la vita è dura (difficult) - le condizioni climatiche in Islanda sono dure (harsh) - la copertina di un libro è dura (hard)