diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6bc8d37..ba0ced9 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -37,6 +37,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - [L10NSharp.Windows.Forms] Restored project-local Resources support for `FallbackLanguagesDlgBase` button images (`Move`, `Move_up`, and `Move_down`).
 - [L10NSharp.Windows.Forms] Corrected resource manager base name to `L10NSharp.Windows.Forms.Properties.Resources`.
 - [L10NSharp.Windows.Forms.Tests] Corrected resource manager base name to `L10NSharp.Windows.Forms.Tests.Properties.Resources`.
+- [L10NSharp] Fixed `FilenamesToAddToCache` yielding both the custom and installed XLIFF for the same language when `UseLanguageCodeFolders` is `true`, causing custom translations to be silently overwritten by installed ones. (#140)
 - [L10NSharp] Fixed `ExtractXliff` accumulating duplicate "Not found in static scan" notes on successive runs; the note is now replaced rather than appended, and removed when the string is subsequently found. (#113)
 - [L10NSharp] Fixed `LocalizationManager.GetString` silently falling back to English when called with a one-shot `IEnumerable<string>` for `preferredLanguageIds`; the sequence is now materialized before use. (#139)
 
diff --git a/src/L10NSharp.Tests/LocalizationManagerTestsBase.cs b/src/L10NSharp.Tests/LocalizationManagerTestsBase.cs
index e312e1d..13d8bd2 100644
--- a/src/L10NSharp.Tests/LocalizationManagerTestsBase.cs
+++ b/src/L10NSharp.Tests/LocalizationManagerTestsBase.cs
@@ -13,7 +13,7 @@
 
 namespace L10NSharp.Tests
 {
-	public abstract class LocalizationManagerTestsBase<T> where T: IDocument
+	public abstract class LocalizationManagerTestsBase<T> where T : IDocument
 	{
 		protected const string AppId = "test";
 		protected const string AppName = "unit test";
@@ -406,39 +406,25 @@ public void GetUiLanguages_FindsAll()
 		[Test]
 		public void GetUiLanguages_FindsAllWithFolders()
 		{
-			try
-			{
-				LocalizationManager.UseLanguageCodeFolders = true;
-				using (var folder = new TempFolder())
-				{
-					SetupManager(folder);
-					var cultures = new List<L10NCultureInfo>(LocalizationManager.GetUILanguages(true));
-					Assert.AreEqual(4, cultures.Count);
-					CollectionAssert.AreEquivalent(new[] { "ar", "en", "fr", "es" }, cultures.Select(c => c.IetfLanguageTag).ToArray());
-					Assert.That(cultures, Is.Ordered.By("DisplayName"));
-				}
-			}
-			finally
+			LocalizationManager.UseLanguageCodeFolders = true;
+			using (var folder = new TempFolder())
 			{
-				LocalizationManager.UseLanguageCodeFolders = false;
+				SetupManager(folder);
+				var cultures = new List<L10NCultureInfo>(LocalizationManager.GetUILanguages(true));
+				Assert.AreEqual(4, cultures.Count);
+				CollectionAssert.AreEquivalent(new[] { "ar", "en", "fr", "es" }, cultures.Select(c => c.IetfLanguageTag).ToArray());
+				Assert.That(cultures, Is.Ordered.By("DisplayName"));
 			}
 		}
 
 		[Test]
 		public void GetDynamicStringInEnglish_NoDefault_FindsEnglishWithFolders()
 		{
-			try
-			{
-				LocalizationManager.UseLanguageCodeFolders = true;
-				using (var folder = new TempFolder())
-				{
-					SetupManager(folder);
-					Assert.That(LocalizationManager.GetDynamicString(AppId, "blahId", null), Is.EqualTo("blah"), "With no default supplied, should find saved English");
-				}
-			}
-			finally
+			LocalizationManager.UseLanguageCodeFolders = true;
+			using (var folder = new TempFolder())
 			{
-				LocalizationManager.UseLanguageCodeFolders = false;
+				SetupManager(folder);
+				Assert.That(LocalizationManager.GetDynamicString(AppId, "blahId", null), Is.EqualTo("blah"), "With no default supplied, should find saved English");
 			}
 		}
 
@@ -474,6 +460,50 @@ public void GetString_OverloadThatTakesListOfLanguages_WorksWithFolders(IEnumera
 			}
 		}
 
+		/// <summary>
+		/// When UseLanguageCodeFolders is true and both an installed and a user-modified file
+		/// exist for the same language, the custom (user-modified) file must win.
+		/// </summary>
+		[Test]
+		public void CustomTranslation_TakesPrecedenceOverInstalled_WithLanguageCodeFolders()
+		{
+			LocalizationManager.UseLanguageCodeFolders = true;
+			using (var folder = new TempFolder())
+			{
+				// Set up the English default in the installed directory.
+				AddEnglishTranslation(GetInstalledDirectory(folder), "1.0");
+
+				// Set up an installed Arabic translation with value "inArabic".
+				AddArabicTranslation(GetInstalledDirectory(folder));
+
+				// Set up a user-modified Arabic translation for the same "theId" string
+				// with a different value to confirm the custom one wins.
+				var customArabicDoc = CreateNewDocument(null, "en", "ar");
+				var customTu = CreateTransUnit("theId", false,
+					CreateTransUnitVariant("en", "wrong"),
+					CreateTransUnitVariant("ar", "custom arabic translation"),
+					"Test", TranslationStatus.Approved);
+				customArabicDoc.AddTransUnit(customTu);
+				var customArabicPath = Path.Combine(GetUserModifiedDirectory(folder),
+					LocalizationManager.GetTranslationFileNameForLanguage(AppId, "ar"));
+				Directory.CreateDirectory(Path.GetDirectoryName(customArabicPath));
+				customArabicDoc.Save(customArabicPath);
+
+				// Set up the localization manager.
+				var manager = CreateLocalizationManager(AppId, AppName, AppVersion,
+					GetInstalledDirectory(folder), GetGeneratedDirectory(folder),
+					GetUserModifiedDirectory(folder));
+				LocalizationManagerInternal<T>.LoadedManagers[AppId] = manager;
+
+				LocalizationManager.SetUILanguage("ar");
+
+				// SUT: the custom translation must win over the installed one.
+				Assert.AreEqual("custom arabic translation",
+					LocalizationManager.GetString("theId", "default"),
+					"Custom (user-modified) translation should take precedence over installed translation when UseLanguageCodeFolders is true");
+			}
+		}
+
 		[Test]
 		public void GetUiLanguages_AzeriHasHackedNativeName()
 		{
diff --git a/src/L10NSharp/XLiffUtils/XliffLocalizationManager.cs b/src/L10NSharp/XLiffUtils/XliffLocalizationManager.cs
index 9fc5b55..361562a 100644
--- a/src/L10NSharp/XLiffUtils/XliffLocalizationManager.cs
+++ b/src/L10NSharp/XLiffUtils/XliffLocalizationManager.cs
@@ -369,8 +369,11 @@ public IEnumerable<string> FilenamesToAddToCache
 							if (string.IsNullOrEmpty(langId) || langId == LocalizationManager.kDefaultLang)
 								continue;
 
-							langIdsOfCustomizedLocales.Add(langId);
-							yield return xliffFile;
+							if (!langIdsOfCustomizedLocales.Contains(langId))
+							{
+								langIdsOfCustomizedLocales.Add(langId);
+								yield return xliffFile;
+							}
 						}
 					}
 					else