title: Tableview with custom cells gets duplicate while scrolling
body: <p>I've created an <code>UITableView</code> based <strong>"count-down"</strong> timer app, each cell has a <strong>"countdown label"</strong> and a <strong>"start running button"</strong>
  The issue is when I press the button, trigger the timer to run,
  then I scroll down, the 10th cell gets trigger too.
  when I scroll back, the first cell timer gets reset.</p>
  
  <p>After searched it might be the <code>dequeueReusableCell</code> part,
  but I don't know how to fix it.</p>
  
  <p>all code on github:
  <a href="https://github.com/evancohi/CountdownTimer" rel="nofollow noreferrer">https://github.com/evancohi/CountdownTimer</a></p>
  
  <p>Here is how my <code>cellForRowAt</code> method setup</p>
  
  <pre><code> func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -&gt; UITableViewCell {
  
       let cell = tableView.dequeueReusableCell(withIdentifier: "timerListCellId", for: indexPath) as! TimerListCell
  
       cell.timerLabel.text = self.timerArray[indexPath.row]
       cell.missionLabel.text = self.missionArray[indexPath.row]
  
       return cell
   }
  </code></pre>
  
  <p>Can you please give some advices?</p>