Errors: Call removeEventListener before addEventListener

plugins/errors.js

@@ -1312,6 +1312,12 @@
 				return;
 			}
 
+			var rEL;
+			if (functionName === "addEventListener") {
+				// grab the native
+				rEL = that.removeEventListener;
+			}
+
 			that[functionName] = function() {
 				try {
 					var args = Array.prototype.slice.call(arguments);
@@ -1336,6 +1342,14 @@
 							// if the callback is already tracked, we won't call addEventListener
 							return;
 						}
+						if (rEL) {
+							// Remove the listener before adding it back in.
+							// This takes care of the (pathological) case where code is relying on the native
+							// de-dupping that the browser provides and BOOMR instruments `addEventListener` between
+							// their redundant calls to `addEventListener`.
+							// We detach with the native because there's no point in calling our wrapped version.
+							rEL.apply(targetObj, arguments);
+						}
 					}
 
 					return origFn.apply(targetObj, args);

tests/page-templates/14-errors/41-addEventListener-dedupping.html

@@ -0,0 +1,27 @@
+<%= header %>
+<%= boomerangDelayedSnippet %>
+<script src="./41-addEventListener-dedupping.js" type="text/javascript"></script>
+<script>
+  function handler() {
+	BOOMR.appendVar("foo", "bar");
+  }
+  function attach() {
+	window.addEventListener("foo", handler);
+  }
+
+  BOOMR_test.init({
+	testAfterOnBeacon: 1,
+	Errors: {
+		enabled: true,
+		monitorEvents: true,
+		sendAfterOnload: true
+	},
+	onBoomerangLoaded: function() {
+		attach();
+		window.dispatchEvent(new Event("foo"));
+	}
+  });
+
+  attach();
+</script>
+<%= footer %>

tests/page-templates/14-errors/41-addEventListener-dedupping.js

@@ -0,0 +1,10 @@
+/*eslint-env mocha*/
+
+describe("e2e/14-errors/41-addEventListener-dedupping", function() {
+	var tf = BOOMR.plugins.TestFramework;
+	it("Should have only fired the foo handler once", function(done) {
+		var b = tf.lastBeacon();
+		assert.equal(b.foo, "bar");
+		done();
+	});
+});