Prevent double-loading stories

When revealing new "pages" before all previous stories finished loading,
the effect hook would initiate multiple requests for the same story.

We now keep track of which requests are in flight/progress to make sure
we only send one request per story.

src/components/List.tsx

@@ -53,16 +53,20 @@ function List({ keys }: Props) {
 
   const slice = useMemo(() => keys.slice(0, size), [keys, size]);
 
+  const loading = useSet<number>();
+
   useEffect(() => {
     slice
-      .filter((key) => !(key in data))
+      .filter((key) => !(key in data) && !loading.has(key))
       .forEach((key) => {
         queue.add(async () => {
           const result = await get(`item/${key}.json`);
           setData((prev) => ({ ...prev, [key]: result }));
         });
+
+        loading.add(key);
       });
-  }, [size]);
+  }, [data, loading, slice]);
 
   const selection = useSet<number>();
   const [cursor, setCursor] = useState(undefined);