Also while we're on this: while every GM asks this question, it covers a huge range of possibilities, e.g.
- "this person is a safety risk and I do not want to be exposed to them"
- "I don't get along with this person and I would rather not interact with them"
- "this person's roleplaying style is incompatible with mine / they are dead wood and will kill all my plots".
- (A subset of the above) "I can't do romance / other specified plot type with this person"
Usually all the GM gets is a name, which can make it difficult to assess which end of that scale a casting conflict is at. Players might want to consider giving a bit more than a name in some cases e.g. if someone is a safety risk, then most GMs want to know this so they can throw them out, because no-one wants that sort of person around. Likewise specific plot-types, because that's the easiest thing to cast around (you might not be able to do a romance plot with someone, but they can totally be your obnoxious sibling).
As a GM, my primary interests in preventing casting conflicts are running a safe game and making the game work. If plots don't trigger because players can't stand the sight of each other, that's less fun for everyone. At the same time, in small games, or when three or more people name the same person, it can be difficult or impossible to avoid clashes, and people will just have to cope.