diff --git a/app/src/main/java/eu/kanade/tachiyomi/ui/library/LibraryController.kt b/app/src/main/java/eu/kanade/tachiyomi/ui/library/LibraryController.kt
index 8916b9c51..b91fb9ed1 100644
--- a/app/src/main/java/eu/kanade/tachiyomi/ui/library/LibraryController.kt
+++ b/app/src/main/java/eu/kanade/tachiyomi/ui/library/LibraryController.kt
@@ -585,6 +585,25 @@ class LibraryController(
                 favSyncDialog?.dismiss()
                 favSyncDialog = null
             }
+            is FavoritesSyncStatus.BadLibraryState.MangaInMultipleCategories -> {
+                releaseSyncLocks()
+
+                favSyncDialog?.dismiss()
+                favSyncDialog = buildDialog()
+                        ?.title("Favorites sync error")
+                        ?.content(status.message + " Sync will not start until the gallery is in only one category.")
+                        ?.cancelable(false)
+                        ?.positiveText("Show gallery")
+                        ?.onPositive { _, _ ->
+                            openManga(status.manga)
+                            presenter.favoritesSync.status.onNext(FavoritesSyncStatus.Idle())
+                        }
+                        ?.negativeText("Ok")
+                        ?.onNegative { _, _ ->
+                            presenter.favoritesSync.status.onNext(FavoritesSyncStatus.Idle())
+                        }
+                        ?.show()
+            }
             is FavoritesSyncStatus.Error -> {
                 releaseSyncLocks()
 
diff --git a/app/src/main/java/exh/favorites/FavoritesSyncHelper.kt b/app/src/main/java/exh/favorites/FavoritesSyncHelper.kt
index 679136e00..960d3a976 100644
--- a/app/src/main/java/exh/favorites/FavoritesSyncHelper.kt
+++ b/app/src/main/java/exh/favorites/FavoritesSyncHelper.kt
@@ -69,6 +69,24 @@ class FavoritesSyncHelper(val context: Context) {
             return
         }
 
+        // Validate library state
+        status.onNext(FavoritesSyncStatus.Processing("Verifying local library"))
+        val libraryManga = db.getLibraryMangas().executeAsBlocking()
+        val seenManga = HashSet<Long>(libraryManga.size)
+        libraryManga.forEach {
+            if(it.source != EXH_SOURCE_ID && it.source != EH_SOURCE_ID) return@forEach
+
+            if(it.id in seenManga) {
+                val inCategories = db.getCategoriesForManga(it).executeAsBlocking()
+                status.onNext(FavoritesSyncStatus.BadLibraryState
+                        .MangaInMultipleCategories(it, inCategories))
+                logger.w("Manga %s is in multiple categories!", it.id)
+                return
+            } else {
+                seenManga += it.id!!
+            }
+        }
+
         //Download remote favorites
         val favorites = try {
             status.onNext(FavoritesSyncStatus.Processing("Downloading favorites from remote server"))
@@ -387,6 +405,11 @@ class FavoritesSyncHelper(val context: Context) {
 sealed class FavoritesSyncStatus(val message: String) {
     class Error(message: String) : FavoritesSyncStatus(message)
     class Idle : FavoritesSyncStatus("Waiting for sync to start")
+    sealed class BadLibraryState(message: String) : FavoritesSyncStatus(message) {
+        class MangaInMultipleCategories(val manga: Manga,
+                                        val categories: List<Category>):
+                BadLibraryState("The gallery: ${manga.title} is in more than one category (${categories.joinToString { it.name }})!")
+    }
     class Initializing : FavoritesSyncStatus("Initializing sync")
     class Processing(message: String, isThrottle: Boolean = false) : FavoritesSyncStatus(if(isThrottle)
         "$message\n\nSync is currently throttling (to avoid being banned from ExHentai) and may take a long time to complete."
diff --git a/app/src/main/java/exh/favorites/LocalFavoritesStorage.kt b/app/src/main/java/exh/favorites/LocalFavoritesStorage.kt
index d34bfb549..96e526a48 100644
--- a/app/src/main/java/exh/favorites/LocalFavoritesStorage.kt
+++ b/app/src/main/java/exh/favorites/LocalFavoritesStorage.kt
@@ -115,14 +115,17 @@ class LocalFavoritesStorage {
                     token = EHentaiSearchMetadata.galleryToken(it.second.url)
                     category = it.first
 
-                    // TODO Throw error here
-                    if(this.category > 9)
+                    if(this.category > MAX_CATEGORIES)
                         return@mapNotNull null
                 }
             }
 
     private fun validateDbManga(manga: Manga)
             = manga.favorite && (manga.source == EH_SOURCE_ID || manga.source == EXH_SOURCE_ID)
+
+    companion object {
+        const val MAX_CATEGORIES = 9
+    }
 }
 
 data class ChangeSet(val added: List<FavoriteEntry>,