diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5dda21c32..1b1dbfc78 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -21,6 +21,7 @@ The format is a modified version of [Keep a Changelog](https://keepachangelog.co
 - Fix next chapter button occasionally jumping to the last page of the current chapter ([@perokhe](https://github.com/perokhe)) ([#1920](https://github.com/mihonapp/mihon/pull/1920))
   - Fix page number not appearing when opening chapter ([@perokhe](https://github.com/perokhe)) ([#1936](https://github.com/mihonapp/mihon/pull/1936))
 - Fix backup sharing from notifications not working when app is in background ([@JaymanR](https://github.com/JaymanR))([#1929](https://github.com/mihonapp/mihon/pull/1929))
+- Fix mark existing duplicate read chapters as read option not working in some cases ([@AntsyLich](https://github.com/AntsyLich)) ([#1944](https://github.com/mihonapp/mihon/pull/1944))
 
 ## [v0.18.0] - 2025-03-20
 ### Added
diff --git a/app/src/main/java/eu/kanade/tachiyomi/ui/reader/ReaderViewModel.kt b/app/src/main/java/eu/kanade/tachiyomi/ui/reader/ReaderViewModel.kt
index d3205f793..21edcdd8c 100644
--- a/app/src/main/java/eu/kanade/tachiyomi/ui/reader/ReaderViewModel.kt
+++ b/app/src/main/java/eu/kanade/tachiyomi/ui/reader/ReaderViewModel.kt
@@ -15,7 +15,6 @@ import eu.kanade.domain.manga.model.readingMode
 import eu.kanade.domain.source.interactor.GetIncognitoState
 import eu.kanade.domain.track.interactor.TrackChapter
 import eu.kanade.domain.track.service.TrackPreferences
-import eu.kanade.tachiyomi.data.database.models.isRecognizedNumber
 import eu.kanade.tachiyomi.data.database.models.toDomainChapter
 import eu.kanade.tachiyomi.data.download.DownloadManager
 import eu.kanade.tachiyomi.data.download.DownloadProvider
@@ -147,6 +146,11 @@ class ReaderViewModel @JvmOverloads constructor(
 
     private var chapterToDownload: Download? = null
 
+    private val unfilteredChapterList by lazy {
+        val manga = manga!!
+        runBlocking { getChaptersByMangaId.await(manga.id, applyScanlatorFilter = false) }
+    }
+
     /**
      * Chapter list for the active manga. It's retrieved lazily and should be accessed for the first
      * time in a background thread to avoid blocking the UI.
@@ -559,15 +563,14 @@ class ReaderViewModel @JvmOverloads constructor(
             .contains(LibraryPreferences.MARK_DUPLICATE_CHAPTER_READ_EXISTING)
         if (!markDuplicateAsRead) return
 
-        val duplicateUnreadChapters = chapterList
-            .mapNotNull {
-                val chapter = it.chapter
+        val duplicateUnreadChapters = unfilteredChapterList
+            .mapNotNull { chapter ->
                 if (
                     !chapter.read &&
                     chapter.isRecognizedNumber &&
-                    chapter.chapter_number == readerChapter.chapter.chapter_number
+                    chapter.chapterNumber.toFloat() == readerChapter.chapter.chapter_number
                 ) {
-                    ChapterUpdate(id = chapter.id!!, read = true)
+                    ChapterUpdate(id = chapter.id, read = true)
                 } else {
                     null
                 }