scripts.js

var Firebase = require('firebase');
var EmailService = require('./services/email');
var ref = new Firebase(process.env.FIREBASE_URL);
var firstLoad = true;
 
ref.authWithCustomToken(process.env.FIREBASE_SECRET, function (err) {
  if (firstLoad) {
    firstLoad = false;
 
    // When a potential tenant submits an application, email out an alert
    ref.child('applicants').on('child_added', function (snapshot) {
        var applicant = snapshot.val();
        if (!(applicant.alerts && applicant.alerts.apply)) {
          var body = applicant.firstName + ' ' + applicant.lastName + '\n' + applicant.email + '\n' + applicant.phoneNumber;
          EmailService
          .send('New Applicant', 'alerts@domain.com', body)
          .then(function () {
            ref.child('applicants').child(snapshot.key()).child('alerts').child('apply').set(true);
          });
        }
    });
  }
});

You said that if (!(applicant.alerts && applicant.alerts.apply)) guards against dupes — what is this output?

Also — have you tried putting a breakpoint/debugger on 18 and going through the stacktrace via node inspector/webstorm?