diff --git a/app/assets/javascripts/discourse/app/controllers/topic.js b/app/assets/javascripts/discourse/app/controllers/topic.js
index 3832df9372a..2e28cca50f1 100644
--- a/app/assets/javascripts/discourse/app/controllers/topic.js
+++ b/app/assets/javascripts/discourse/app/controllers/topic.js
@@ -890,7 +890,8 @@ export default Controller.extend(bufferedProperty("model"), {
     selectReplies(post) {
       ajax(`/posts/${post.id}/reply-ids.json`).then((replies) => {
         const replyIds = replies.map((r) => r.id);
-        this.selectedPostIds.pushObjects([post.id, ...replyIds]);
+        const postIds = [...this.selectedPostIds, post.id, ...replyIds];
+        this.set("selectedPostIds", [...new Set(postIds)]);
         this._forceRefreshPostStream();
       });
     },
diff --git a/app/assets/javascripts/discourse/tests/unit/controllers/topic-test.js b/app/assets/javascripts/discourse/tests/unit/controllers/topic-test.js
index e7b66504a9d..de0517afb08 100644
--- a/app/assets/javascripts/discourse/tests/unit/controllers/topic-test.js
+++ b/app/assets/javascripts/discourse/tests/unit/controllers/topic-test.js
@@ -599,6 +599,52 @@ discourseModule("Unit | Controller | topic", function (hooks) {
     );
   });
 
+  test("selectReplies", async function (assert) {
+    pretender.get("/posts/1/reply-ids.json", () => {
+      return [
+        200,
+        { "Content-Type": "application/json" },
+        [{ id: 2, level: 1 }],
+      ];
+    });
+
+    let model = topicWithStream({
+      posts: [{ id: 1 }, { id: 2 }],
+    });
+
+    const controller = this.getController("topic", { model });
+
+    controller.send("selectReplies", { id: 1 });
+    await settled();
+
+    assert.strictEqual(
+      controller.get("selectedPostsCount"),
+      2,
+      "It should select two, the post and its replies"
+    );
+
+    controller.send("togglePostSelection", { id: 1 });
+    assert.strictEqual(
+      controller.get("selectedPostsCount"),
+      1,
+      "It should be selecting one only "
+    );
+    assert.strictEqual(
+      controller.get("selectedPostIds")[0],
+      2,
+      "It should be selecting the reply id "
+    );
+
+    controller.send("selectReplies", { id: 1 });
+    await settled();
+
+    assert.strictEqual(
+      controller.get("selectedPostsCount"),
+      2,
+      "It should be selecting two, even if reply was already selected"
+    );
+  });
+
   test("topVisibleChanged", function (assert) {
     let model = topicWithStream({
       posts: [{ id: 1 }],