I know that the effects of a spell don’t combine if it is cast multiple times. Only the most potent effect would be applied. In our last game, this topic came up in regards to the Harm spell. Harm was cast on an opponent. He failed the save, took a lot of damage and his hit point maximum was reduced drastically. A turn later, he got slammed again with Harm. This time, he succeeded on the save and very little necrotic damage was dealt. Then, the discussion started whether his hit point maximum would now be further reduced or if the “more potent” Harm spell takes precedence.
Naturally, I argued that his hit point maximum would not be reduced further, because effects of the same spell cast multiple times don’t stack. The counterargument was, that the necrotic damage dealt by the Harm spell is clearly an effect of the spell. And no one would argue, that the damage dealt by two consecutive Harm spells would only be applied once. So now I ask you:
Can the hit point maximum be reduced repeatedly with multiple Harm spells?
Best Answer
The target's maximum hit points will only be reduced once
From Harm (PHB, pg. 249):
Combining Magical Effects, from the PHB (pg. 254):
So multiple castings of the same spell will not stack, but rather the one that dealt the largest amount of damage would take precedence. This rule applies because the hit point reduction has a duration attached to it, unlike, say, a Wraith's Life Drain (MM, pg. 302), which is an ongoing affect (albeit with an end condition):
Of course, this doesn't stop the target's maximum hit points from being reduced further by something other than the Harm spell...
This is essentially the argument you presented at your table, but I see no reason that this wouldn't apply.