diff --git a/core/misc/ajax.js b/core/misc/ajax.js
index b63bc03..53e3c77 100644
--- a/core/misc/ajax.js
+++ b/core/misc/ajax.js
@@ -427,15 +427,19 @@
       ajax.options.data.dialogOptions = element_settings.dialog;
     }
 
-    // Ensure that we have a valid URL by adding ? when no query parameter is
-    // yet available, otherwise append using &.
-    if (ajax.options.url.indexOf('?') === -1) {
-      ajax.options.url += '?';
+    // Append WRAPPER_FORMAT to the URL as a query parameter or replace it's
+    // value if it is already present.
+    var queryParameter = Drupal.ajax.WRAPPER_FORMAT + '=drupal_' + (element_settings.dialogType || 'ajax');
+    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 {
-      ajax.options.url += '&';
+      var regexPattern = new RegExp(Drupal.ajax.WRAPPER_FORMAT + '=[^&]*', 'i');
+      ajax.options.url = ajax.options.url.replace(regexPattern, queryParameter);
     }
-    ajax.options.url += Drupal.ajax.WRAPPER_FORMAT + '=drupal_' + (element_settings.dialogType || 'ajax');
 
     // Bind the ajaxSubmit function to the element event.
     $(ajax.element).on(element_settings.event, function (event) {