diff --git a/app/assets/javascripts/discourse/app/controllers/composer.js b/app/assets/javascripts/discourse/app/controllers/composer.js
index 36139cffd39..81cfa6af1c6 100644
--- a/app/assets/javascripts/discourse/app/controllers/composer.js
+++ b/app/assets/javascripts/discourse/app/controllers/composer.js
@@ -649,27 +649,30 @@ export default class ComposerController extends Controller {
 
         const [linkWarn, linkInfo] = linkLookup.check(post, href);
 
-        if (linkWarn && !this.get("isWhispering")) {
-          let body;
+        if (linkWarn && !this.isWhispering) {
           if (linkInfo.username === this.currentUser.username) {
-            body = I18n.t("composer.duplicate_link_same_user", {
-              domain: linkInfo.domain,
-              post_url: topic.urlForPostNumber(linkInfo.post_number),
-              ago: shortDate(linkInfo.posted_at),
+            this.appEvents.trigger("composer-messages:create", {
+              extraClass: "custom-body",
+              templateName: "education",
+              body: I18n.t("composer.duplicate_link_same_user", {
+                domain: linkInfo.domain,
+                post_url: topic.urlForPostNumber(linkInfo.post_number),
+                ago: shortDate(linkInfo.posted_at),
+              }),
             });
           } else {
-            body = I18n.t("composer.duplicate_link", {
-              domain: linkInfo.domain,
-              username: linkInfo.username,
-              post_url: topic.urlForPostNumber(linkInfo.post_number),
-              ago: shortDate(linkInfo.posted_at),
+            this.appEvents.trigger("composer-messages:create", {
+              extraClass: "custom-body duplicate-link-message",
+              templateName: "education",
+              body: I18n.t("composer.duplicate_link", {
+                domain: linkInfo.domain,
+                username: linkInfo.username,
+                post_url: topic.urlForPostNumber(linkInfo.post_number),
+                ago: shortDate(linkInfo.posted_at),
+              }),
             });
           }
-          this.appEvents.trigger("composer-messages:create", {
-            extraClass: "custom-body",
-            templateName: "education",
-            body,
-          });
+
           return false;
         }
       }
diff --git a/app/assets/javascripts/discourse/tests/acceptance/composer-messages-test.js b/app/assets/javascripts/discourse/tests/acceptance/composer-messages-test.js
index 8d78ac5114a..aab3fdc86bb 100644
--- a/app/assets/javascripts/discourse/tests/acceptance/composer-messages-test.js
+++ b/app/assets/javascripts/discourse/tests/acceptance/composer-messages-test.js
@@ -3,9 +3,16 @@ import {
   exists,
   query,
 } from "discourse/tests/helpers/qunit-helpers";
-import { click, fillIn, triggerKeyEvent, visit } from "@ember/test-helpers";
+import {
+  click,
+  fillIn,
+  triggerKeyEvent,
+  visit,
+  waitUntil,
+} from "@ember/test-helpers";
 import { test } from "qunit";
 import I18n from "I18n";
+import pretender, { response } from "../helpers/create-pretender";
 
 acceptance("Composer - Messages", function (needs) {
   needs.user();
@@ -22,13 +29,14 @@ acceptance("Composer - Messages", function (needs) {
   test("Shows warning in composer if user hasn't been seen in a long time.", async function (assert) {
     await visit("/u/charlie");
     await click("button.compose-pm");
-    assert.ok(
-      !exists(".composer-popup"),
+    assert.false(
+      exists(".composer-popup"),
       "composer warning is not shown by default"
     );
+
     await triggerKeyEvent(".d-editor-input", "keyup", "Space");
-    assert.ok(exists(".composer-popup"), "shows composer warning message");
-    assert.ok(
+    assert.true(exists(".composer-popup"), "shows composer warning message");
+    assert.true(
       query(".composer-popup").innerHTML.includes(
         I18n.t("composer.user_not_seen_in_a_while.single", {
           usernames: ['<a class="mention" href="/u/charlie">@charlie</a>'],
@@ -60,13 +68,14 @@ acceptance("Composer - Messages - Cannot see group", function (needs) {
   test("Shows warning in composer if group hasn't been invited", async function (assert) {
     await visit("/t/130");
     await click("button.create");
-    assert.ok(
-      !exists(".composer-popup"),
+    assert.false(
+      exists(".composer-popup"),
       "composer warning is not shown by default"
     );
+
     await fillIn(".d-editor-input", "Mention @staff");
-    assert.ok(exists(".composer-popup"), "shows composer warning message");
-    assert.ok(
+    assert.true(exists(".composer-popup"), "shows composer warning message");
+    assert.true(
       query(".composer-popup").innerHTML.includes(
         I18n.t("composer.cannot_see_group_mention.not_allowed", {
           group: "staff",
@@ -79,13 +88,14 @@ acceptance("Composer - Messages - Cannot see group", function (needs) {
   test("Shows warning in composer if group hasn't been invited, but some members have access already", async function (assert) {
     await visit("/t/130");
     await click("button.create");
-    assert.ok(
-      !exists(".composer-popup"),
+    assert.false(
+      exists(".composer-popup"),
       "composer warning is not shown by default"
     );
+
     await fillIn(".d-editor-input", "Mention @staff2");
-    assert.ok(exists(".composer-popup"), "shows composer warning message");
-    assert.ok(
+    assert.true(exists(".composer-popup"), "shows composer warning message");
+    assert.true(
       query(".composer-popup").innerHTML.includes(
         I18n.t("composer.cannot_see_group_mention.some_not_allowed", {
           group: "staff2",
@@ -96,3 +106,54 @@ acceptance("Composer - Messages - Cannot see group", function (needs) {
     );
   });
 });
+
+acceptance("Composer - Messages - Duplicate links", function (needs) {
+  needs.user();
+
+  test("Shows the warning", async function (assert) {
+    pretender.get("/inline-onebox", () =>
+      response({
+        "inline-oneboxes": [],
+      })
+    );
+
+    let receivedMessages = false;
+    pretender.get("/composer_messages", () => {
+      receivedMessages = true;
+      return response({
+        composer_messages: [],
+        extras: {
+          duplicate_lookup: {
+            "test.localhost/t/testing-topic/123/4567": {
+              domain: "test.localhost",
+              username: "uwe_keim",
+              posted_at: "2021-01-01T12:00:00.000Z",
+              post_number: 1,
+            },
+          },
+        },
+      });
+    });
+
+    await visit("/t/internationalization-localization/280");
+    await click("button.create");
+
+    // Work around the lack of CSS transitions in the test env
+    query("#reply-control").dispatchEvent(new Event("transitionend"));
+
+    assert
+      .dom(".composer-popup")
+      .doesNotExist("composer warning is not shown by default");
+
+    await waitUntil(() => receivedMessages);
+
+    await fillIn(
+      ".d-editor-input",
+      "Here's a link: https://test.localhost/t/testing-topic/123/4567"
+    );
+
+    assert
+      .dom(".composer-popup.duplicate-link-message")
+      .exists("shows composer warning message");
+  });
+});
diff --git a/app/controllers/composer_messages_controller.rb b/app/controllers/composer_messages_controller.rb
index 3785e257288..893a30dec5e 100644
--- a/app/controllers/composer_messages_controller.rb
+++ b/app/controllers/composer_messages_controller.rb
@@ -22,7 +22,6 @@ class ComposerMessagesController < ApplicationController
     usernames = params.require(:usernames)
     users = ComposerMessagesFinder.user_not_seen_in_a_while(usernames)
     user_count = users.count
-    warning_message = nil
 
     if user_count > 0
       message_locale =