diff --git a/src/Transport/Sendmail.php b/src/Transport/Sendmail.php
index 3223b3fa..98ee2142 100644
--- a/src/Transport/Sendmail.php
+++ b/src/Transport/Sendmail.php
@@ -222,12 +222,7 @@ protected function prepareBody(Mail\Message $message)
      */
     protected function prepareHeaders(Mail\Message $message)
     {
-        // On Windows, simply return verbatim
-        if ($this->isWindowsOs()) {
-            return $message->getHeaders()->toString();
-        }
-
-        // On *nix platforms, strip the "to" header
+        // Strip the "to" and "subject" headers
         $headers = clone $message->getHeaders();
         $headers->removeHeader('To');
         $headers->removeHeader('Subject');
diff --git a/test/Transport/SendmailTest.php b/test/Transport/SendmailTest.php
index abdd3787..111f8f3b 100644
--- a/test/Transport/SendmailTest.php
+++ b/test/Transport/SendmailTest.php
@@ -28,6 +28,7 @@ class SendmailTest extends TestCase
     public $message;
     public $additional_headers;
     public $additional_parameters;
+    public $operating_system;
 
     public function setUp()
     {
@@ -72,9 +73,14 @@ public function getMessage()
         return $message;
     }
 
+    private function isWindows()
+    {
+        return $this->operating_system === 'WIN';
+    }
+
     public function testReceivesMailArtifactsOnUnixSystems()
     {
-        if ($this->operating_system == 'WIN') {
+        if ($this->isWindows()) {
             $this->markTestSkipped('This test is *nix-specific');
         }
 
@@ -96,7 +102,7 @@ public function testReceivesMailArtifactsOnUnixSystems()
 
     public function testReceivesMailArtifactsOnWindowsSystems()
     {
-        if ($this->operating_system != 'WIN') {
+        if (! $this->isWindows()) {
             $this->markTestSkipped('This test is Windows-specific');
         }
 
@@ -120,7 +126,7 @@ public function testReceivesMailArtifactsOnWindowsSystems()
 
     public function testLinesStartingWithFullStopsArePreparedProperlyForWindows()
     {
-        if ($this->operating_system != 'WIN') {
+        if (! $this->isWindows()) {
             $this->markTestSkipped('This test is Windows-specific');
         }
 
@@ -255,4 +261,25 @@ public function testDoNotAllowMessageWithoutToAndCcAndBccHeaders()
         $this->expectException(RuntimeException::class);
         $this->transport->send($message);
     }
+
+    /**
+     * @see https://github.com/laminas/laminas-mail/issues/19
+     */
+    public function testHeadersToAndSubjectAreNotDuplicated()
+    {
+        $message = new Message();
+        $message
+            ->addTo('matthew@example.org')
+            ->addFrom('ralph@example.org')
+            ->setSubject('Greetings and Salutations!')
+            ->setBody("Sorry, I'm going to be late today!");
+
+        $this->transport->send($message);
+
+        $this->assertEquals('matthew@example.org', $this->to);
+        $this->assertEquals('Greetings and Salutations!', $this->subject);
+
+        $this->assertNotRegExp('/^To: matthew\@example\.org$/m', $this->additional_headers);
+        $this->assertNotRegExp('/^Subject: Greetings and Salutations!$/m', $this->additional_headers);
+    }
 }