title: Receiving: 'Error [ERR_HTTP_HEADERS_SENT] Cannot set headers warning', but data is posting to table?
body: <p>I have an app.post that is receiving this error message:</p>
  
  <p><strong>Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client</strong></p>
  
  <p>...but data is publishing to the MySQL Table.</p>
  
  <p>What is happening? Where is it happeneing? How can I resolve this issue?</p>
  
  <pre><code> app.post("/api/tracking/retrieve", (req, res) =&gt; {
  
      res.setHeader('Content-Type', 'application/json');
  
      let carrierTrackers = [
          {
              carrier: 'USPS',
              trackingCode: '940019999999'
          },
          {
              carrier: 'UPS',
              trackingCode: '94001888888'
          }
      ];
  
      carrierTrackers.forEach((carrierTracker) =&gt; {
          const tracker = new api.Tracker({
              tracking_code: carrierTracker.trackingCode,
              carrier: carrierTracker.carrier
          })
  
          tracker
              .save()
              .then(
                  console.log(carrierTracker.carrier, carrierTracker.trackingCode),
          )
              .catch(error =&gt; {
                  console.log('There has been an error with your submission.')
              });
  
          table = 'tracking_table';
          col = ['user_id', 'tracking_number'];
          val = [user_id, tracker.tracking_code];
  
          main.create(table, col, val, function (data) {
              res.send(JSON.stringify({
                  id: "",
                  user_id: user_id,
                  tracking_number: data.tracking_code
              })); 
          })
          res.end()
      })
  });
  </code></pre>