Checking for two equal values in objects, in an array

You can transform your list of objects to be mapped by their start attribute and check to see if any of those dates contain more than one object.

Here's your helper:

function transform(list) {
  var map = {};
  for (var i = 0; i < list.length; i++) {
    var object = list[i];
    if (object.start in map) {
      map[object.start].push(object);
    } else {
      map[object.start] = [];
      map[object.start].push(object);
    }
  }
  return object;
}

Then all you need to do is call transform with your list of objects to get back your map of objects identified by their start dates. Iterate through them and you'll get all of the objects for each start date.

var startDates = transform(list); // your list of objects mapped by the start date
// loop through each start date
for (var startDate in startDates) {
  var listForStartDate = startDates[startDate];

  // check to see if more than one object exists for this start date
  if (listForStartDate.length > 1) {
    // do whatever you want with your list of objects for the current start date
  }
}

First loop through the array, then loop again from that point on, check to see if any of the starts are the same...

for (var i = 0; i < array.length; i++) {
  var currentObj = array[i];
  for (var j = i + 1; j < array.length; j++) {
    var other = array[j];
    if (currentObj.start === other.start) {
      // do stuff here
    }
  }
}

I would first use something like Lodash's _.groupBy() to create an object where the key is the date (as a formatted string or string of the numeric value) and the value is an array of objects with the same date.

Then iterate over the object and if the value array has more than one element, do something with it.

e.g. if using Lodash (assuming your array of objects is called array):

var grouped = _.groupBy(array, function (obj) { return '' + obj.start.getTime(); });
_.each(grouped, function (objects, date) {
    if (objects.length > 1) {
        doSomething(objects);
    }
});

If not using Lodash/Underscore, you can just roll your own grouping function:

function groupBy(collection, callback) {
    var output = {};
    angular.forEach(collection, function (obj) {
        var groupKey = callback(obj);
        if (!output[groupKey]) {
            output[groupKey] = [obj];
        } else {
            output[groupKey].push(obj);
        }
    });
    return output;
}

// to use it:

var groupedByDate = groupBy(array, function (obj) { return '' + obj.start.getTime(); });
angular.forEach(groupedByDate, function (objects, date) {
    if (objects.length > 1) {
        doSomething(objects);
    }
});