FIX: Memory leak when adding ajax prefilter repeatedly.

app/assets/javascripts/discourse/initializers/csrf-token.js.es6

@@ -1,16 +1,21 @@
 //  Append our CSRF token to AJAX requests when necessary.
+
+let installedFilter = false;
+
 export default {
   name: "csrf-token",
   initialize: function(container) {
-    var session = container.lookup("session:main");
-
     // Add a CSRF token to all AJAX requests
+    let session = container.lookup("session:main");
     session.set("csrfToken", $("meta[name=csrf-token]").attr("content"));
 
+    if (!installedFilter) {
       $.ajaxPrefilter(function(options, originalOptions, xhr) {
         if (!options.crossDomain) {
           xhr.setRequestHeader("X-CSRF-Token", session.get("csrfToken"));
         }
       });
+      installedFilter = true;
+    }
   }
 };