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						- # inflight
 - 
 - Add callbacks to requests in flight to avoid async duplication
 - 
 - ## USAGE
 - 
 - ```javascript
 - var inflight = require('inflight')
 - 
 - // some request that does some stuff
 - function 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 called
 - req('foo', cb1)
 - req('foo', cb2)
 - req('foo', cb3)
 - req('foo', cb4)
 - ```
 
 
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