diff --git a/Lib/email/encoders.py b/Lib/email/encoders.py
index 55741a22a07b204..0977a26ff597566 100644
--- a/Lib/email/encoders.py
+++ b/Lib/email/encoders.py
@@ -30,6 +30,7 @@ def encode_base64(msg):
     orig = msg.get_payload(decode=True)
     encdata = str(_bencode(orig), 'ascii')
     msg.set_payload(encdata)
+    del msg['Content-Transfer-Encoding']
     msg['Content-Transfer-Encoding'] = 'base64'
 
 
@@ -41,6 +42,7 @@ def encode_quopri(msg):
     orig = msg.get_payload(decode=True)
     encdata = _qencode(orig)
     msg.set_payload(encdata)
+    del msg['Content-Transfer-Encoding']
     msg['Content-Transfer-Encoding'] = 'quoted-printable'
 
 
diff --git a/Lib/test/test_email/test_email.py b/Lib/test/test_email/test_email.py
index e40c82bba9af426..81f9fe9b680d226 100644
--- a/Lib/test/test_email/test_email.py
+++ b/Lib/test/test_email/test_email.py
@@ -916,6 +916,20 @@ def test_default_cte(self):
         msg = MIMEText('hello \xf8 world', _charset='iso-8859-1')
         eq(msg['content-transfer-encoding'], 'quoted-printable')
 
+    def test_encode_replaces_existing_cte(self):
+        # gh-43702: re-encoding must replace the old Content-Transfer-Encoding
+        # header, not add a second one.
+        for charset, encoder, cte in (
+                ('iso-8859-1', encoders.encode_base64, 'base64'),
+                ('utf-8', encoders.encode_quopri, 'quoted-printable')):
+            with self.subTest(cte=cte):
+                msg = MIMEText('h\xe9llo', _charset=charset)
+                encoder(msg)
+                self.assertEqual(msg.get_all('content-transfer-encoding'),
+                                 [cte])
+                self.assertEqual(msg.get_payload(decode=True),
+                                 'h\xe9llo'.encode(charset))
+
     def test_encode7or8bit(self):
         # Make sure a charset whose input character set is 8bit but
         # whose output character set is 7bit gets a transfer-encoding
diff --git a/Misc/NEWS.d/next/Library/2026-09-26-10-43-18.gh-issue-43702.koAZST.rst b/Misc/NEWS.d/next/Library/2026-09-26-10-43-18.gh-issue-43702.koAZST.rst
new file mode 100644
index 000000000000000..abe4b48a29ace8d
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-09-26-10-43-18.gh-issue-43702.koAZST.rst
@@ -0,0 +1,4 @@
+Fix :func:`email.encoders.encode_base64` and
+:func:`email.encoders.encode_quopri` adding a second
+:mailheader:`Content-Transfer-Encoding` header when they re-encode a
+message that already has one.