|                                     |  | # inflight
Add callbacks to requests in flight to avoid async duplication
## USAGE
```javascriptvar inflight = require('inflight')
// some request that does some stufffunction req(key, callback) {  // key is any random string.  like a url or filename or whatever.  //  // will return either a falsey value, indicating that the  // request for this key is already in flight, or a new callback  // which when called will call all callbacks passed to inflightk  // with the same key  callback = inflight(key, callback)
  // If we got a falsey value back, then there's already a req going  if (!callback) return
  // this is where you'd fetch the url or whatever  // callback is also once()-ified, so it can safely be assigned  // to multiple events etc.  First call wins.  setTimeout(function() {    callback(null, key)  }, 100)}
// only assigns a single setTimeout// when it dings, all cbs get calledreq('foo', cb1)req('foo', cb2)req('foo', cb3)req('foo', cb4)```
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