I would agree with RachelG that you'll probably be OK, but be prepared as there is a chance someone will be a stickler and make you gate check the bag. If that happens, it's best to have quick access to any items that may be fragile or needed during the flight--even if you just place those items in a grocery bag near the top, that should be good enough as you may need to grab them quickly and hand the duffel back to the agent for checking. Again, you'll probably be fine, but better to be prepared just in case.