The Magic Initiate Feat has a restriction of "Once you cast [this spell], you must finish a long rest before you can cast it again". Some have argued that this prevents you from casting the spell more than once per long rest, even if you can already cast the spell from some other feature.
This made me wonder if this line of reasoning is applicable elsewhere. Were there any other situations where being able to cast a spell from multiple sources imposed a net restriction on your ability to cast it? I came up with the following scenario to demonstrate.
Take for example, a multiclass Shadow Monk/Arcane Trickster who has learned the Darkness spell from their Arcane Trickster subclass.
The Shadow Monk's description says:
PHB 80: As an action, you can spend 2 ki points to cast darkness, darkvision, pass without trace, or silence, without providing material components.
The Arcane Trickster's description says:
PHB 98 To cast one of these spells, you must expend a slot of the spell's level or higher
Therefore, if the character spends 2 ki points to cast Darkness, and Darkness is one of their Arcane Trickster spells, they must also spend a spell slot on top of the 2 ki points.
Is there any flaw in my reasoning? This conclusion seems nonsensical to me, but if you accept the above interpretation of Magic Initiate, is there any reason to rule otherwise?