You can get rewards from both Koan and Duncan in the Quest "Doing Duncan a
favor". Once you refuse to kill Koan in the desert during the dialog you can
still kill him using a weapon and retrieve the cube to Duncan and get the
experience reward.

How do you think this should be resolved?

We could remove experience points for killing Koan (there is a way to detect if
a NPC has died) after refusing to do so, although I'm not entirely certain how
that would work if the player leveled up. Alternatively, we could not award
experience points when Duncon receives the cube.

I think we need to move the reward for not killing Koan to be given by Duncan
upon learning that Tux spared Koan. That would resolve the quest and make it
impossible to get both rewards.

Tania will already try to kill Tux if he kill Koan. Isn't that enough?

No, I had solved the Duncan quest the non-evil way and already rescued Tania. I
returned now to the desert, killed Koan with a plasma disc, picked up the cube
and gave it to Duncan, earning the experience reward. Tania stayed friendly and
healed my wounds afterwards in Ewald's bar. There is a similar issue with the
miner: you can get his lamp for Iris, and threaten him to obtain his information.

Maybe change the priority to lesser than release-blocker. This sequence has been
possible for some time.
Resolution may not be so easy to resolve at this time with current NPC model/code.

Cool bug. Try to set a spared_koan var, 
and ignore the cube if you have that var.
Or drop the cube only by Lua code, instead 
of having it dropped on RoT file.
Or say that this is a feature for smart 
pkayers who know how to get maximum 
rewards, as long that getting both doesnt 
affect gameplay.
For Iris... Only show if you was friendly 
with the miner? Instead of not displaying 
if you was hostile.

Issue is with Koan/Duncan but Tux's interaction with Iris/Bruce keeps being
brought up. Start a seperate BUG!!!!

I would appreciate a feedback on my 
suggestions. Can anyone do this?

I think the Duncan quest needs a complete rewrite. Currently, the player can get the 
reward only if he has never met Koan. Then, the player can only choose between 
outright evil or spare naively. Finally, Tux never get a clue about Koan or the 
cube.

In my opinion, the correct solution is to expand the quest by allowing Tux to help 
Koan. By example, faking the death of Koan. Or providing Koan with some means of 
defense. In any case, Tux must have terminated the quest with Duncan before he 
receive the reward of Koan.

So, I prefer to spend more time of the quest after the 0.16 release. I think we can 
live with this bug. If everybody agree, I will lower the priority.

I agree, this is the best solution and is 
not for 0.16. I would mark as Important 
if asked.

Xenux - concur. Downgrade from release-blocker.
Bug is assigned to you so I'm hesitant to change its status.