diff --git a/app/build.gradle b/app/build.gradle
index 16552e455..9db761d08 100644
--- a/app/build.gradle
+++ b/app/build.gradle
@@ -82,7 +82,7 @@ afterEvaluate {
 }
 dependencies {
 	//noinspection GradleDependency
-	implementation('com.github.KotatsuApp:kotatsu-parsers:0aa4ea01f7') {
+	implementation('com.github.KotatsuApp:kotatsu-parsers:b7613606c0') {
 		exclude group: 'org.json', module: 'json'
 	}
 
diff --git a/app/src/main/kotlin/org/koitharu/kotatsu/reader/domain/ChapterPages.kt b/app/src/main/kotlin/org/koitharu/kotatsu/reader/domain/ChapterPages.kt
index 7175f53f2..c86159033 100644
--- a/app/src/main/kotlin/org/koitharu/kotatsu/reader/domain/ChapterPages.kt
+++ b/app/src/main/kotlin/org/koitharu/kotatsu/reader/domain/ChapterPages.kt
@@ -14,6 +14,7 @@ class ChapterPages private constructor(private val pages: ArrayDeque<ReaderPage>
 	val chaptersSize: Int
 		get() = indices.size()
 
+	@Synchronized
 	fun removeFirst() {
 		val chapterId = pages.first().chapterId
 		indices.remove(chapterId)
@@ -25,6 +26,7 @@ class ChapterPages private constructor(private val pages: ArrayDeque<ReaderPage>
 		shiftIndices(delta)
 	}
 
+	@Synchronized
 	fun removeLast() {
 		val chapterId = pages.last().chapterId
 		indices.remove(chapterId)
@@ -33,17 +35,28 @@ class ChapterPages private constructor(private val pages: ArrayDeque<ReaderPage>
 		}
 	}
 
-	fun addLast(id: Long, newPages: List<ReaderPage>) {
+	@Synchronized
+	fun addLast(id: Long, newPages: List<ReaderPage>): Boolean {
+		if (id in indices) {
+			return false
+		}
 		indices.put(id, pages.size until (pages.size + newPages.size))
 		pages.addAll(newPages)
+		return true
 	}
 
-	fun addFirst(id: Long, newPages: List<ReaderPage>) {
+	@Synchronized
+	fun addFirst(id: Long, newPages: List<ReaderPage>): Boolean {
+		if (id in indices) {
+			return false
+		}
 		shiftIndices(newPages.size)
 		indices.put(id, newPages.indices)
 		pages.addAll(0, newPages)
+		return true
 	}
 
+	@Synchronized
 	fun clear() {
 		indices.clear()
 		pages.clear()
@@ -58,7 +71,7 @@ class ChapterPages private constructor(private val pages: ArrayDeque<ReaderPage>
 		return pages.subList(range.first, range.last + 1)
 	}
 
-	operator fun contains(chapterId: Long)  = indices.contains(chapterId)
+	operator fun contains(chapterId: Long) = chapterId in indices
 
 	private fun shiftIndices(delta: Int) {
 		for (i in 0 until indices.size()) {