IME // +RangeReplaceableCollection <- ctlPrefWindow.

Source/Modules/IMEModules/IME.swift

@@ -225,3 +225,14 @@ import Cocoa
 	}
 
 }
+
+// MARK: - Root Extensions
+
+// Extend the RangeReplaceableCollection to allow it clean duplicated characters.
+// Ref: https://stackoverflow.com/questions/25738817/
+extension RangeReplaceableCollection where Element: Hashable {
+	var charDeDuplicate: Self {
+		var set = Set<Element>()
+		return filter { set.insert($0).inserted }
+	}
+}

Source/WindowControllers/ctlPrefWindow.swift

@@ -27,14 +27,6 @@ CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 import Carbon
 import Cocoa
 
-// Extend the RangeReplaceableCollection to allow it clean duplicated characters.
-extension RangeReplaceableCollection where Element: Hashable {
-	var charDeDuplicate: Self {
-		var set = Set<Element>()
-		return filter { set.insert($0).inserted }
-	}
-}
-
 // Please note that the class should be exposed using the same class name
 // in Objective-C in order to let IMK to see the same class name as
 // the "InputMethodServerPreferencesWindowControllerClass" in Info.plist.