My kerbal visited orbit of almost every planet orbit, (Moho, Eve, Duna and even Jool but the on Mun and Minmus is the only place that i allowed them l'm little a bit like overprotective parent, a little bit like mu Mum:D But i afraid that after landing i will not have enough fuel to bring them back to Kerbin, because landing cost considerable amount of dV and i'm rather lousy at math:) And i don't want strand my kerbal in space, it strange but i feel affection to all my crews. Even IRL nor NASA nor Roscoms made sample return mission from Mars or any other planet of our solar system, because it's so difficult. But i think about solution than can work for Duna(it's quite long time since i played ksp, due my university study, i started play again after release of 0.24) I think that i can send fuel tank, a probe with big fuel tank to Duna orbit, than send Kerbals to land on Duna, i didn't know how to build Mars, oh yeah Duna lander:D (thus i'm not know how such thing should look) and after mission the crew(actually i'm) will not have to worry about fuel because it's already here on Duna orbit. PS. I'm wonder does NASA consider similar tactic for real life mission to Mars?