Can't believe I missed this one. You can sell Quan's holy weapon in gen 1, and buy it with someone else. Since Altena has that same item added to her inventory in gen 2 (presumably because Quan can lose it when he's a green unit), you can end up with two copies of the same item.

Easiest solution would be to simply not give Quan his holy weapon in gen 1, since by time he gets it there are only pirates to kill, and the arena if it's not cleared. But that's a deviation from vanilla, so it might be better to treat the weapon like Sigurd's holy weapon - add it to the excluded inherited items list, and blacklist all other units from getting the same major holy blood as Quan.

There is one lingering question with the latter method, however. Can Sigurd and Quan be allowed to have the same holy blood? The potential issue comes from chapter 5, when Quan appears as a green unit. If someone else has his holy weapon from earlier, and you do a save and reset, what happens? I'm inclined to believe nothing will get removed, since enemies in gen 2 that are given drops don't lose them if you already have those items. The only reason I can think of that those two couldn't have the same holy blood is if the weapon held by a player unit gets lost, which I don't think would happen. I can verify it in a couple of days once I reach chapter 5.