diff --git a/core/misc/ajax.js b/core/misc/ajax.js
index 194c5e5..858f4b3 100644
--- a/core/misc/ajax.js
+++ b/core/misc/ajax.js
@@ -248,17 +248,21 @@
       ajax.options.data.dialogOptions = elementSettings.dialog;
     }

-    if (ajax.options.url.indexOf('?') === -1) {
-      ajax.options.url += '?';
-    } else {
-      ajax.options.url += '&';
-    }
-
     var wrapper = 'drupal_' + (elementSettings.dialogType || 'ajax');
     if (elementSettings.dialogRenderer) {
       wrapper += '.' + elementSettings.dialogRenderer;
     }
-    ajax.options.url += Drupal.ajax.WRAPPER_FORMAT + '=' + wrapper;
+
+    var queryParameter = Drupal.ajax.WRAPPER_FORMAT + '=' + wrapper;
+    if (ajax.options.url.indexOf(Drupal.ajax.WRAPPER_FORMAT) === -1) {
+      // Ensure that we have a valid URL by adding ? when no query parameter is
+      // yet available, otherwise append using &.
+      ajax.options.url += (ajax.options.url.indexOf('?') === -1) ? '?' : '&';
+      ajax.options.url += queryParameter;
+    } else {
+      var regexPattern = new RegExp(Drupal.ajax.WRAPPER_FORMAT + '=[^&]*', 'i');
+      ajax.options.url = ajax.options.url.replace(regexPattern, queryParameter);
+    }

     $(ajax.element).on(elementSettings.event, function (event) {
       if (!drupalSettings.ajaxTrustedUrl[ajax.url] && !Drupal.url.isLocal(ajax.url)) {