#Make a new general question and answer it - then mark those other questions as a duplicate of that instead
Make a new general question and answer it - then mark those other questions as a duplicate of that instead
As Kamil Drakari said in their answer, most of the questions marked as a duplicate of the wizard spellbook question are not exactly a duplicate of the wizard one, but they are very related to one another. I agree with Kamil that the aspect of the wizard question involving copying spells into their spellbook makes it slightly different from most of those marked as a duplicate of it (though the "Assuming I can do that, can I cast such a spell" part covers the same ground).
I think the best solution would probably be to ask a general question about what spell level multiclassed spellcasters can learn or prepare spells at, and then provide an answer that covers that general rule. Then it can serve as the destination of the various duplicates of that question for specific builds.
###Update (July 2019)
Update (July 2019)
I've gone ahead and posted the question, as I suggested: If I multiclass into 2 or more spellcasting classes, how do I determine my known/prepared spells?
I also went ahead and updated some of the old questions that had been incorrectly marked as a duplicate of the wizard question to point to my new question instead - though I didn't go searching for any others (if at least to avoid flooding the front page all at once).