double_callback.js

var request = require('supertest');
var express = require('express');

var app = express();
app.get('/something', function (req, res) { res.send(200); });

it('should respond with 200', function (done) {
  // PRODUCES DOUBLE CALLBACK
  request(app)
    .get('/something')
    .expect(200, function (res) {
      return false;
    })
    .end(done);
});

it('should also respond with 200', function (done) {
  // DOES NOT PRODUCE DOUBLE CALLBACK
  request(app)
    .get('/something')
    .expect(function (res) {
      return false;
    })
    .expect(200, done);
});