Reminder: Information is not shared between your characters!
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It's pretty plain and simple: You cannot make a character, learn something, then make a new character and claim your characters know each other and share that knowledge. We have a word for that, it's called "Metagaming," and it comes with consequences.
There are "grey areas," such as "I learned something, made a new character and someone else told that character what my original character said!" The best rule to follow is if you are purposefully seeking out those "grey areas" you're just as guilty of metagaming as if you had just skipped that step to begin with.