diff --git a/app/locations/trailing-history.js b/app/locations/trailing-history.js
index 0cb8dfb..675484e 100644
--- a/app/locations/trailing-history.js
+++ b/app/locations/trailing-history.js
@@ -10,6 +10,8 @@ export default HistoryLocation.extend({
 
 export function formatURL(url) {
   let modifiedURL = new URL(url, 'http://example.com');
-  modifiedURL.pathname += '/';
+  if (!modifiedURL.pathname.endsWith('/')) {
+    modifiedURL.pathname += '/';
+  }
   return `${modifiedURL.pathname}${modifiedURL.search}${modifiedURL.hash}`;
 }
diff --git a/tests/unit/locations/trailing-history-test.js b/tests/unit/locations/trailing-history-test.js
index c4bb8bf..1e6a01f 100644
--- a/tests/unit/locations/trailing-history-test.js
+++ b/tests/unit/locations/trailing-history-test.js
@@ -13,6 +13,10 @@ module('Unit | Locations | trailing history', function (hooks) {
     assert.deepEqual(formatURL('/foo#placeholder'), '/foo/#placeholder');
   });
 
+  test('does not add double slashes', function (assert) {
+    assert.deepEqual(formatURL('/foo/'), '/foo/');
+  });
+
   test('supports query params and anchors', function (assert) {
     assert.deepEqual(
       formatURL('/foo?bar=baz#placeholder'),