So, in an ideal world, either incoming players to a game will have a look at the current cast to make sure what they make doesn't step on anyone's thematic toes, or if that doesn't happen the narrator will review the character and compare it to what's already on the table to make sure everything gels. Or better yet, both!

But, since we're human, that doesn't always happen, and what does happen sometimes is that a new character comes in who ends up covering the same sort of thematic ground as an existing character–which, to me, feels like stealing someone else's thunder, and that's not really cool. What I suggest, to help mitigate this, is that narrator-created player cards, i.e. pregenned strengths, weaknesses, natures and subplots be exclusive to a single player character. So when one person creates a character with a given card, that card is removed from the stable of prewritten choices for incoming players. This won't prevent every instance of the problem outlined above, but it should hopefully cut down on a significant portion of casual breaches of thematic exclusivity.

It could perhaps at least be an optional switch for narrators to set, if not as a hardcoded thing, as I can see cases where two PCs having the same card could be a good thing for certain types of games (perhaps especially with larger casts).