For single-target buffs that can be put on several targets at once, you can't click them off using the hotkey button. That just casts a new spell, and I believe if you don't have enough concentration slots it cancels the first one cast to make room. You have to click (or sometimes right-click and select cancel from the menu) the icon in the maintained spells window to cancel the one on a specific target (the tooltip identifies the target).
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