Implement dedupeInFlight(fetch)

Implement dedupeInFlight(fetch)

A component re-rendering, or two independent pieces of UI, can both ask for the same URL at the same moment. Write dedupeInFlight(fetch) that returns a wrapped fetch: if a request to a given url is already in flight, a second call to the same url returns the same pending promise instead of issuing a second real request. Once that request settles, the next call to the same url starts a fresh request.

const deduped = dedupeInFlight(fetch);
const [a, b] = await Promise.all([deduped("/data"), deduped("/data")]);
// only ONE real request was made, a and b are the same response