title: Hangfire - Prevent multiples of the same job being enqueued
body: <h2>Scenario:</h2>
  
  <p><strong>Job 1</strong> is scheduled to run every 5 minutes, and takes ~1 minute to complete.</p>
  
  <p>A lot of work piles up and <strong>Job 1</strong> takes 15 minutes to run.</p>
  
  <p>There are now three <strong>Job 1's</strong> being processed concurrently - I don't want this.
  <hr>
  How do I prevent <strong>Job 1</strong> being added to the queue again if it is already there?</p>
  
  <p>Is there Hangfire setting, or do I need to poll job statuses manually?</p>