index.js

var output = [];
//var dataSource = app.models.Employee.newQuery().run();
var dataSource = [
  {
    timestamp: "2017-09-12 13:49",
    fullname: "John",
    emailAddress: "john@doe.com"
    //....
  },
  {
    timestamp: "2017-06-12 13:49",
    fullname: "John",
    emailAddress: "john@doe.com"
  },
  {
    timestamp: "2017-09-12 13:49",
    fullname: "Johhyn",
    emailAddress: "john@dosse.com"
  }
];

for (var i = 0; i < dataSource.length; i++) {
  var employee = dataSource[i];

  var foundOutputEmployeeIndex = null;

  for (var j = 0; j < output.length; j++) {
    if (
      output[j].emailAddress === employee.emailAddress &&
      foundEmp.timestamp < employee.timestmap
    ) {
      output[j] = employee;
    }
  }

  if (foundOutputEmployeeIndex != null) {
    output.push(employee);
  }
}

console.log(output);