A duffle bag will work, of course, but it's one more thing to carry for the entire trip.
There is a FREE OPTION -- Many international airlines (and I believe ALL the ones that belong to the Star Alliance) give out huge, heavy duty plastic bags for people transporting golf clubs, baby carriers, etc. I have used them with great success. I can fit my Ghost backpack, water bottles and hiking poles, etc. and still with tons of room to spare. The United Airline ones weigh 2.6oz. each.
A second option is to pack everything into your carry-on size backpack, except for your hiking poles and Swiss Army knife. You check those using a mailing tube -- like this:
The main benefit is that you minimize the chance of your entire backpack getting lost enroute. After all, if your hiking poles and knife get lost, you can still continue on with your trip. Anyway, food for thought.