Adjust how item quest encounters select item to give

So, I've been on an item quest that just expired.  I needed 3 items.  So far, I've gotten one of the items via encounters NINE times.  I've gotten the second item FIVE times.  I've gotten the third item, ZERO times.  The item I needed was the opossum carcass, which as far as I can tell can only be obtained from the lunar event encounters otherwise.  Which I got the encounter for that about six times, but never actually rolled go give me the item from it, and the event is over,  so I can't even keep grinding for it.  I get that sometimes RNG can be against you, but since some items are literally impossible to get otherwise, could we somehow give slightly better odds to the item NOT currently sitting in a large pile in the hoard rather than amassing an ever growing but ultimately less useful collection of the same one or two items over and over?  I get encouraging people to buy from other players, but when these encounters already have a chance of giving nothing anyway, I think this level is a bit too unbalanced.  Slightly giving preference to the item not already obtained will make it actually possible to complete quests on your own if you play frequently enough.  I often fail item collection quests because of not getting items from encounters, or not getting encounters at all, which I accept as a risk of the game, but this is one of the few times I've been able to get a lot of items, but still not complete it because the one single item is avoiding me.

As far as what needs to be added/changed, I'm not a coder, but I'm sure there is a way to modify the odds of which item is given based on what's available in the hoard.