diff --git a/flask/sessions.py b/flask/sessions.py
index 2dbd8b32..9d937be0 100644
--- a/flask/sessions.py
+++ b/flask/sessions.py
@@ -405,7 +405,7 @@ class SecureCookieSessionInterface(SessionInterface):
 
         # Add a "Vary: Cookie" header if the session was accessed at all.
         if session.accessed:
-            response.headers.add('Vary', 'Cookie')
+            self._patch_vary_cookie_header(response)
 
         if not self.should_set_cookie(app, session):
             return
@@ -423,3 +423,16 @@ class SecureCookieSessionInterface(SessionInterface):
             path=path,
             secure=secure
         )
+
+    def _patch_vary_cookie_header(self, response):
+        """
+        Add a 'Cookie' value to the 'Vary' header if one is not already present.
+        """
+        header = response.headers.get('Vary', '')
+        headers = [h.strip() for h in header.split(',') if h]
+
+        if not any(h.lower() == 'cookie' for h in headers):
+            headers.append('Cookie')
+
+        updated_header = ', '.join(headers)
+        response.headers['Vary'] = updated_header
diff --git a/tests/test_basic.py b/tests/test_basic.py
index de56cb43..e4d6b2f9 100644
--- a/tests/test_basic.py
+++ b/tests/test_basic.py
@@ -520,15 +520,31 @@ def test_session_vary_cookie(app, client):
     def setdefault():
         return flask.session.setdefault('test', 'default')
 
+    @app.route('/vary-cookie-header-set')
+    def vary_cookie_header_set():
+        response = flask.Response()
+        response.headers['Vary'] = 'Cookie'
+        flask.session['test'] = 'test'
+        return response
+
+    @app.route('/vary-header-set')
+    def vary_header_set():
+        response = flask.Response()
+        response.headers['Vary'] = 'Accept-Encoding, Accept-Language'
+        flask.session['test'] = 'test'
+        return response
+
     @app.route('/no-vary-header')
     def no_vary_header():
         return ''
 
-    def expect(path, header=True):
+    def expect(path, header_value='Cookie'):
         rv = client.get(path)
 
-        if header:
-            assert rv.headers['Vary'] == 'Cookie'
+        if header_value:
+            # The 'Vary' key should exist in the headers only once.
+            assert len(rv.headers.get_all('Vary')) == 1
+            assert rv.headers['Vary'] == header_value
         else:
             assert 'Vary' not in rv.headers
 
@@ -536,7 +552,9 @@ def test_session_vary_cookie(app, client):
     expect('/get')
     expect('/getitem')
     expect('/setdefault')
-    expect('/no-vary-header', False)
+    expect('/vary-cookie-header-set')
+    expect('/vary-header-set', 'Accept-Encoding, Accept-Language, Cookie')
+    expect('/no-vary-header', None)
 
 
 def test_flashes(app, req_ctx):