fix: Avoid preloading the resource multiple times

[shuding]

In a real-world application, a “preload” action might be initiated from multiple different places and levels (global, inside component, inside hooks, inside event callbacks). Hence deduplication is also needed here.

This improves use cases such as:

<button onHover={() => preload('/api/user?id=' + userId, fetcher)}>

Where you can hover the button multiple times. Also:

function App({ userId }) {
  useEffect(() => {
    preload('/api/user?id=' + userId, fetcher)
  }, [userId])
  ...
}

Where there can be multiple <App/> elements.

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[koba04]

This looks good! Thank you!👍