Duplication Bug w/ 1.0.11v :
So... this is a bug of course and all you need to do is have the @Tinkers' Construct 'Napsack'
All that happens is that what ever you have in the bag will stay there when you log in and out of a world. Try it for your self!
I have not tested this with multiplayer because i cannot find a good server to do so. If you want, please test this on multiplayer and report back to me
I tested the bug with a lot of items from all different mods and It all works the same.