You might expect we could just have the spell itself keep a list of monsters it has hit so far, and check the list when an arrow hits a monster to see if that monster has already been hit. And that is indeed possible. However, having a bunch of code that controls a spell with a central data structure that every aspect of the spell reports back to is not a very good model. It leads to confusing, hard-to-change code.
A different approach would be that the multishot spell creates 20 arrows, and gives them initial velocity, and then that's it, the spell is gone. The arrows are now all separate. There are two ways to avoid redundant hits now. Either the arrows can have a list of all the other arrows and send a message when they hit something so the others know not to damage it (alternatively arrows could have access to everything and then search through everything for the other arrows from the spell. As long as they are all uniquely marked as coming from that spell they could be found that way). Or option two is to have the monster keep track of what it's been hit by. Then when an arrow hits a monster, it can check that monster's list of multi-shot spells that already hit it to see if it should do damage.
This is interesting due to parallels with capitalism and with physics.
The capitalism parallel is that autonomous, smart agents are a better model than central control. Programmers have known this for decades (at least Lisp programmers!), and write lots of papers about it. Here is a paper on dividing problems up into smaller discrete tasks, with detailed examples, which shows how this makes programs easier to modify. It explicitly criticises trying to code single, large functions that keep track of everything, and criticises programming languages that encourage or require that. Similarly, capitalists know (and have since before Lisp existed) that central authorities don't work as well as distributed decision making. Another point in the Lisp paper I linked is that lazy evaluation making is very valuable. That means only calculating things when they are about to be used to prevent doing unnecessary calculations that might not be used. Similarly, when people make their own choices, they can frequently do it at the last moment, and they can avoid deciding things that become irrelevant. When central planners try to plan, they have to, in order to have time to tell everyone what to do, plan way in advance, so they end up calculating lots of things that, it turns out, don't matter.
The parallel with physics is that a huge amount of mysticism can be detected and refuted by a detailed analysis of information flow. For example, suppose someone claimed a certain arrow would only hurt you if you hadn't already been hit by another arrow from the same group. We would know there is no overall central control mechanism (located at the bow?) that arrows report back to (what do they report back with? light? we'd notice, and light has limited speed so it wouldn't work if the arrows went too fast). We also know arrows don't pass messages to each other about what targets they've hit (not only are arrows unable to identify what people they've hit, they don't have anything to send or receive messages with). And we know the model of the monster keeping a list of which arrows it was hit by wouldn't work either (that involved the arrow, on hitting something, checking the list, but arrows cannot do that. how would it read the list? compute whether it was on the list? also arrows aren't marked by what volley they were fired in). So we can call anyone who believes in a real multishot spell of this sort a mystic (after we ask his explanation, and it turns out he has no explanation of how his idea is possible within the laws of physics).
This arguing technique applies to a lot more than magic spells. Suppose someone said he spoke to God. We might well ask how the information got from God to him. If God communicated with light or sound it could be recorded with a video camera, and he'd need a convincing reason to think it wasn't just a natural process (there is a lot of light bouncing around. how do you know this light bounced off God?). People who believe in telepathy never explain how thoughts travel between brains, nor what they sense thoughts with (eyes? neurons?). Do psychics who do phone readings claim that reading thoughts is possible from many miles away? If it is, why can't they read the thoughts of people they aren't on the phone with? Do thoughts travel through telephone wires? Of course, phone psychics in fact just don't bother to address the issue at all. It'd be very amusing if they were asked questions like this more often. Some would be foolish enough to attempt to answer some of the questions. It's pretty hard to refuse to say the range of one's psychic powers. But it'd also be pretty embarrassing to claim telephones are a psychic amplifier. And if it's someone's voice that matters, why won't a recording do? And after the psychic says it recordings don't work, trick one and do an entire phone reading by playing pre-recorded sound bites over the phone and then ask why the psychic didn't notice he wasn't talking to a person (shouldn't his powers have not worked?).
It works on a lot of bad philosophy too. Imagine someone says that meaning is assigned to objects by humans, and can exist no other way. That is nonsense. The first thing to do is ask whether its possible to think about something before assigning it a meaning. If that's possible, begin asking about what difference it makes to human thinking whether a meaning has been assigned or not. What specifically, if anything, is impossible before a meaning is assigned? Why does assigning a meaning change that?
The more interesting case is when the person says that thinking about something that your brain hasn't yet assigned a meaning is impossible. Next he will say that meaning is assigned immediately when a person first encounters something, and not before, and not after. One issue is this needs to be done instantly, so that stray thoughts don't try to think about the object before the meaning is assigned. Thinking instantly isn't possible because electrical impulses require time to move around. Further, when the assign-meaning function is called, it needs to know: A) what the object is B) what meaning to assign. All the information necessary to assign the meaning must be there before the meaning is assigned, and thus before the object has been thought about at all. That means all meanings are assigned without thinking about what they should be! But it gets worse. If Jack will respond to seeing a rock for the first time by assigning it "hard" (that's over-simplified), then we might say he already, right now, before interacting with his first rock ... has a worldview such that rocks will be assigned the meaning "hard". So what difference does it make if Jack assigns that meaning now or later? What's so special about the act of assigning when the result could have been worked out in advance? People may say Jack could change his worldview before seeing a rock. But that doesn't really change anything: as he changes his worldview, the implied meaning of rocks according to Jack changes as well. And what about assigning meaning to objects that don't exist anymore but that we've heard of? And objects that don't exist yet but will? Were cars meaningless until they were completely invented?