Ensure that we can add res.marko to multiple express instances, but it will not be applied multiple times to the same express instance.

express.js

@@ -28,3 +28,5 @@ response.marko = response.marko || function(template, data) {
 
     template.render(data, res);
 };
+
+delete require.cache[__filename];

test/express-test.js

@@ -10,17 +10,51 @@ var express = require('express');
 var request = require('request');
 var fs = require('fs');
 
+var markoExpressPath = require.resolve('../express');
+
 require('../node-require').install();
 
-describe('render', function() {
+describe('express', function() {
     var autoTestDir = path.join(__dirname, 'autotests/express');
 
+    describe('registration', function() {
+        it('should not register the res.marko function multiple times', function() {
+            require(markoExpressPath);
+
+            var fn = express.response.marko;
+
+            require(markoExpressPath);
+
+            chai.expect(express.response.marko).to.equal(fn);
+        });
+        it('should be able to register for multiple express instances', function() {
+            // test res.marko is added to the real express response
+            require(markoExpressPath);
+            chai.expect(express.response.marko).to.be.a('function');
+
+            // set up an express mock object and hijack require
+            var expressMock = { response:{} };
+            var _require = module.require;
+
+            module.require = function() {
+                if(arguments[0] === 'express') return expressMock;
+                return _require.apply(module, arguments);
+            };
+
+            // check that res.marko is added to the mocked express response
+            require(markoExpressPath);
+            chai.expect(expressMock.response.marko).to.be.a('function');
+
+            // return require to its original state
+            module.require = _require;
+        });
+    })
+
     autotest.scanDir(
         autoTestDir,
         function run(dir, helpers, done) {
             var mainPath = path.join(dir, 'test.js');
             var templatePath = path.join(dir, 'template.marko');
-            var markoExpressPath = require.resolve('../express');
 
             var main = fs.existsSync(mainPath) ? require(mainPath) : {};
             var loadOptions = main && main.loadOptions;