Question about spellcasters and un-learning spells

Spellcasters won't cast spells autonomously unless they're gathering in the club where one of the activities is casting spells.

And don't worry about the Duplicato spell. It's one of the three ultimate spells (the others being Strangeify and Rite of Ascension), meaning that you won't accidentally learn it, but instead you must learn it from one of the sages (which one depends on the spell).