diff --git a/src/routes/_actions/timeline.js b/src/routes/_actions/timeline.js
index 5d4237ef..4722e234 100644
--- a/src/routes/_actions/timeline.js
+++ b/src/routes/_actions/timeline.js
@@ -10,6 +10,9 @@ import { getStatus, getStatusContext } from '../_api/statuses'
 import { emit } from '../_utils/eventBus'
 import { TIMELINE_BATCH_SIZE } from '../_static/timelines'
 import { timelineItemToSummary } from '../_utils/timelineItemToSummary'
+import uniqBy from 'lodash-es/uniqBy'
+
+const byId = _ => _.id
 
 async function storeFreshTimelineItemsInDatabase (instanceName, timelineName, items) {
   await database.insertTimelineItems(instanceName, timelineName, items)
@@ -70,7 +73,7 @@ export async function addTimelineItemSummaries (instanceName, timelineName, newS
   const oldSummaries = store.getForTimeline(instanceName, timelineName, 'timelineItemSummaries') || []
   const oldStale = store.getForTimeline(instanceName, timelineName, 'timelineItemSummariesAreStale')
 
-  const mergedSummaries = mergeArrays(oldSummaries, newSummaries, compareTimelineItemSummaries)
+  const mergedSummaries = uniqBy(mergeArrays(oldSummaries, newSummaries, compareTimelineItemSummaries), byId)
 
   if (!isEqual(oldSummaries, mergedSummaries)) {
     store.setForTimeline(instanceName, timelineName, { timelineItemSummaries: mergedSummaries })
diff --git a/src/routes/_components/list/List.html b/src/routes/_components/list/List.html
index 1e4eff47..e423917f 100644
--- a/src/routes/_components/list/List.html
+++ b/src/routes/_components/list/List.html
@@ -19,6 +19,7 @@
   import ListLazyItem from './ListLazyItem.html'
   import { listStore } from './listStore'
   import { getScrollContainer } from '../../_utils/scrollContainer'
+  import { observe } from 'svelte-extras'
 
   function getScrollTopOffset () {
     const style = getComputedStyle(document.documentElement)
@@ -30,11 +31,20 @@
     oncreate () {
       const { realm } = this.get()
       this.store.setCurrentRealm(realm)
+
+      if (process.env.NODE_ENV !== 'production') {
+        this.observe('safeItems', safeItems => {
+          if (new Set(safeItems).size !== safeItems.length) {
+            console.error('list of items is not unique:', safeItems)
+          }
+        })
+      }
     },
     ondestroy () {
       this.store.setCurrentRealm(null)
     },
     methods: {
+      observe,
       itemInitialized () {
         let { initializedCount, length } = this.get()
         initializedCount++
diff --git a/tests/spec/122-replies-in-thread.js b/tests/spec/122-replies-in-thread.js
index fbac9064..2fd1dd02 100644
--- a/tests/spec/122-replies-in-thread.js
+++ b/tests/spec/122-replies-in-thread.js
@@ -3,9 +3,17 @@ import {
   getNthStatus,
   getNthStatusContent,
   getUrl,
-  getNthReplyButton, getNthComposeReplyInput, sleep,
+  getNthReplyButton,
+  getNthComposeReplyInput,
+  sleep,
   getAriaSetSize,
-  getNthComposeReplyButton, goBack, goForward
+  getNthComposeReplyButton,
+  goBack,
+  goForward,
+  composeInput,
+  composeButton,
+  getNthStatusId,
+  getStatusContents
 } from '../utils'
 import { postAs, postReplyAs } from '../serverActions'
 
@@ -106,3 +114,71 @@ test('reply to non-focused grandchild status in a thread', async t => {
     .click(getNthComposeReplyButton(3))
   await verifyAriaSetSize(t, '4')
 })
+
+async function replyToNthStatus (t, n, replyText, expectedN) {
+  await t
+    .click(getNthReplyButton(n))
+    .typeText(getNthComposeReplyInput(n), replyText, { paste: true })
+    .click(getNthComposeReplyButton(n))
+    .expect(getNthStatusContent(expectedN).innerText).contains(replyText)
+}
+
+test('no duplicates in threads', async t => {
+  await loginAsFoobar(t)
+  await t
+    .hover(composeInput)
+    .typeText(composeInput, 'this is my thread 1', { paste: true })
+    .click(composeButton)
+    .expect(getNthStatusContent(1).innerText).contains('this is my thread 1')
+    .click(getNthStatus(1))
+    .expect(getUrl()).contains('status')
+  await replyToNthStatus(t, 1, 'this is my thread 2', 2)
+  const [id1, id2] = await Promise.all([getNthStatusId(1)(), getNthStatusId(2)()])
+  await t
+    .click(getNthStatus(2))
+    .expect(getUrl()).contains(id2)
+  await replyToNthStatus(t, 2, 'this is my thread 3', 3)
+  const id3 = await getNthStatusId(3)()
+  await postReplyAs('quux', 'hey i am replying to 1', id1)
+  await postReplyAs('admin', 'hey i am replying to 3', id3)
+  await t
+    .expect(getNthStatusContent(4).innerText).contains('hey i am replying to 3', { timeout: 20000 })
+  const idReplyTo3 = await getNthStatusId(4)()
+  await t
+    .click(getNthStatus(4))
+    .expect(getUrl()).contains(idReplyTo3)
+  await postReplyAs('quux', 'hey check this reply', id1)
+  await t
+    .click(getNthStatus(2))
+    .expect(getUrl()).contains(id2)
+    .click(getNthStatus(3))
+    .expect(getUrl()).contains(id3)
+  await replyToNthStatus(t, 3, 'this is my thread 4', 5)
+  await replyToNthStatus(t, 5, 'this is my thread 5', 6)
+  const id5 = await getNthStatusId(6)()
+  await postReplyAs('admin', 'hey i am replying to 1 again', id1)
+  await t
+    .click(getNthStatus(6))
+    .expect(getUrl()).contains(id5)
+    .click(getNthStatus(1))
+    .expect(getUrl()).contains(id1)
+
+  await t
+    .click(getNthStatus(6))
+    .expect(getUrl()).contains(id5)
+  await replyToNthStatus(t, 5, 'this is my thread 6', 6)
+  await t
+    .click(getNthStatus(1))
+    .expect(getUrl()).contains(id1)
+
+  const contents = await getStatusContents()
+  const contentsToCounts = new Map()
+  for (const content of contents) {
+    contentsToCounts.set(content, 1 + (contentsToCounts.get(content) || 0))
+  }
+  const duplicates = [...contentsToCounts.entries()].filter(arr => arr[1] > 1).map(arr => arr[0])
+
+  // there should be no duplicates
+  await t
+    .expect(duplicates).eql([])
+})
diff --git a/tests/utils.js b/tests/utils.js
index fa822185..2081eb6a 100644
--- a/tests/utils.js
+++ b/tests/utils.js
@@ -167,6 +167,27 @@ export const getActiveElementInsideNthStatus = exec(() => {
   return ''
 })
 
+export const getNthStatusId = n => exec(() => {
+  return document.querySelector(getNthStatusSelector(n))
+    .getAttribute('id')
+    .split('/')
+    .slice(-1)[0]
+}, {
+  dependencies: {
+    getNthStatusSelector,
+    n
+  }
+})
+
+export const getStatusContents = exec(() => {
+  const res = []
+  const elements = document.querySelectorAll('.list-item > article .status-content')
+  for (let i = 0; i < elements.length; i++) {
+    res.push(elements[i].innerText)
+  }
+  return res
+})
+
 export const getTitleText = exec(() => document.head.querySelector('title') && document.head.querySelector('title').innerHTML)
 
 export const goBack = exec(() => window.history.back())