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I see that there is a stack restriction going on here. You cannot have the Urn and Vessel buff or debuff at the same time. Urn's buff gets overwritten by Vessel's buff, and same for the debuffs.
You also do not lose a charge when casting Urn on an already Vessel-buffed ally or already Vessel-debuffed enemy. But the item still goes on cooldown.
Instead of allowing to target these units, Urn of Shadows should not be able to target them in the first place, since it does nothing but waste the cooldown.
Repro:
1. Have an Urn of Shadow with charges, and an ally with Spirit Vessel with charges
2. Have the ally cast Spirit Vessel on an ally or enemy
3. Cast Urn of Shadows on the same target
Result: Your Urn of Shadow does nothing, as it cannot override Spirit Vessel's effect. It doesn't lose charges either. But it still allows targeting that unit, going on cooldown.
Expected: Urn of Shadows should not be able to target units that already have the Spirit Vessel buff or debuff (depending on team of course, it should not be able to target a unit which has been targeted by an allied Vessel).
You also do not lose a charge when casting Urn on an already Vessel-buffed ally or already Vessel-debuffed enemy. But the item still goes on cooldown.
Instead of allowing to target these units, Urn of Shadows should not be able to target them in the first place, since it does nothing but waste the cooldown.
Repro:
1. Have an Urn of Shadow with charges, and an ally with Spirit Vessel with charges
2. Have the ally cast Spirit Vessel on an ally or enemy
3. Cast Urn of Shadows on the same target
Result: Your Urn of Shadow does nothing, as it cannot override Spirit Vessel's effect. It doesn't lose charges either. But it still allows targeting that unit, going on cooldown.
Expected: Urn of Shadows should not be able to target units that already have the Spirit Vessel buff or debuff (depending on team of course, it should not be able to target a unit which has been targeted by an allied Vessel).