Only allow a ID to be registered once.
npm install doormanOnly allow a ID to be registered once.

npm install doorman
``js`
var dedupe = doorman()
dedupe('1') // true
dedupe('1') // false
dedupe('2') // true
dedupe('2') // false
dedupe.list() // { '1': true, '2': true }
`js
var dedupe = doorman({ '1': true })
dedupe('1') // false
dedupe('2') // true
dedupe('2') // false
``