diff --git a/GUI/extenwindow.cpp b/GUI/extenwindow.cpp
index 0c6450d..28e4fb3 100644
--- a/GUI/extenwindow.cpp
+++ b/GUI/extenwindow.cpp
@@ -7,8 +7,12 @@
 #include <QMimeData>
 #include <QUrl>
 #include <QLabel>
+#include <QMessageBox>
 
 extern const char* EXTENSIONS;
+extern const char* INVALID_EXTENSION;
+extern const char* CONFIRM_EXTENSION_OVERWRITE;
+extern const char* EXTENSION_WRITE_ERROR;
 extern const char* EXTEN_WINDOW_INSTRUCTIONS;
 
 namespace
@@ -22,15 +26,19 @@ namespace
 	concurrency::reader_writer_lock extenMutex;
 	std::vector<Extension> extensions;
 
-	void Load(QString extenName)
+	bool Load(QString extenName)
 	{
-		if (extenName == ITH_DLL) return;
 		// Extension is dll and exports "OnNewSentence"
-		if (auto callback = (decltype(Extension::callback))GetProcAddress(LoadLibraryOnce(S(extenName)), "OnNewSentence"))
+		if (QTextFile(extenName + ".dll", QIODevice::ReadOnly).readAll().contains("OnNewSentence"))
 		{
-			std::scoped_lock writeLock(extenMutex);
-			extensions.push_back({ S(extenName), callback });
+			if (auto callback = (decltype(Extension::callback))GetProcAddress(LoadLibraryOnce(S(extenName)), "OnNewSentence"))
+			{
+				std::scoped_lock writeLock(extenMutex);
+				extensions.push_back({ S(extenName), callback });
+				return true;
+			}
 		}
+		return false;
 	}
 
 	void Unload(int index)
@@ -95,14 +103,6 @@ void ExtenWindow::Sync()
 	}
 }
 
-void ExtenWindow::Add(QFileInfo extenFile)
-{
-	if (extenFile.suffix() != "dll") return;
-	QFile::copy(extenFile.absoluteFilePath(), extenFile.fileName());
-	Load(extenFile.completeBaseName());
-	Sync();
-}
-
 bool ExtenWindow::eventFilter(QObject* target, QEvent* event)
 {
 	// See https://stackoverflow.com/questions/1224432/how-do-i-respond-to-an-internal-drag-and-drop-operation-using-a-qlistwidget/1528215
@@ -132,5 +132,16 @@ void ExtenWindow::dragEnterEvent(QDragEnterEvent* event)
 
 void ExtenWindow::dropEvent(QDropEvent* event)
 {
-	for (auto file : event->mimeData()->urls()) Add(file.toLocalFile());
+	for (auto file : event->mimeData()->urls())
+	{
+		QFileInfo extenFile = file.toLocalFile();
+		if (extenFile.suffix() != "dll") continue;
+		if (QFile::exists(extenFile.fileName()) && extenFile.absolutePath() != QDir::currentPath())
+		{
+			if (QMessageBox::question(this, EXTENSIONS, CONFIRM_EXTENSION_OVERWRITE) == QMessageBox::Yes) QFile::remove(extenFile.fileName());
+			if (!QFile::copy(extenFile.absoluteFilePath(), extenFile.fileName())) QMessageBox::warning(this, EXTENSIONS, EXTENSION_WRITE_ERROR);
+		}
+		if (Load(extenFile.completeBaseName())) Sync();
+		else QMessageBox::information(this, EXTENSIONS, QString(INVALID_EXTENSION).arg(extenFile.fileName()));
+	}
 }
diff --git a/GUI/extenwindow.h b/GUI/extenwindow.h
index cc2d1eb..3bd1c3c 100644
--- a/GUI/extenwindow.h
+++ b/GUI/extenwindow.h
@@ -24,7 +24,6 @@ public:
 private:
 	inline static constexpr auto EXTEN_SAVE_FILE = u8"SavedExtensions.txt";
 
-	void Add(QFileInfo extenFile);
 	void Sync();
 	bool eventFilter(QObject* target, QEvent* event) override;
 	void keyPressEvent(QKeyEvent* event) override;
diff --git a/GUI/mainwindow.h b/GUI/mainwindow.h
index cee7ec2..c271d61 100644
--- a/GUI/mainwindow.h
+++ b/GUI/mainwindow.h
@@ -3,6 +3,7 @@
 #include "qtcommon.h"
 #include "extenwindow.h"
 #include "host/host.h"
+#include <QSettings>
 
 namespace Ui
 {
diff --git a/GUI/qtcommon.h b/GUI/qtcommon.h
index 83594b2..5d6f42c 100644
--- a/GUI/qtcommon.h
+++ b/GUI/qtcommon.h
@@ -5,12 +5,9 @@
 #include <QVector>
 #include <QHash>
 #include <QMainWindow>
-#include <QDialog>
 #include <QFile>
 #include <QFileInfo>
 #include <QDir>
-#include <QRegularExpression>
-#include <QSettings>
 
 struct QTextFile : QFile { QTextFile(QString name, QIODevice::OpenMode mode) : QFile(name) { open(mode | QIODevice::Text); } };
 inline std::wstring S(const QString& S) { return { S.toStdWString() }; }
diff --git a/text.cpp b/text.cpp
index 785d0d7..63bdca4 100644
--- a/text.cpp
+++ b/text.cpp
@@ -41,7 +41,11 @@ const char* SAVE_SETTINGS = u8"Save settings";
 const char* EXTEN_WINDOW_INSTRUCTIONS = u8R"(Drag and drop extension (.dll) files here from your computer to add them
 (Does not work if running as administrator)
 Drag and drop within the list to reorder
+(Extensions are used from top to bottom: order DOES matter)
 Press delete with an extension selected to remove it)";
+const char* INVALID_EXTENSION = u8"%1 is an invalid extension";
+const char* CONFIRM_EXTENSION_OVERWRITE = u8"Another version of this extension already exists, do you want to delete and overwrite it?";
+const char* EXTENSION_WRITE_ERROR = u8"Failed to save extension";
 const char* USE_JP_LOCALE = u8"Emulate japanese locale?";
 extern const char* HOOH_SEARCH_UNSTABLE_WARNING = u8"Searching for hooks is unstable! Be prepared for your game to crash!";
 extern const char* SEARCH_PATTERN = u8"Search pattern (hex byte array)";