diff --git a/flask/helpers.py b/flask/helpers.py
index f37be677..2bcdc10b 100644
--- a/flask/helpers.py
+++ b/flask/helpers.py
@@ -1004,3 +1004,17 @@ def is_ip(value):
             return True
 
     return False
+
+
+def patch_vary_header(response, value):
+    """Add a value to the ``Vary`` header if it is not already present."""
+
+    header = response.headers.get('Vary', '')
+    headers = [h for h in (h.strip() for h in header.split(',')) if h]
+    lower_value = value.lower()
+
+    if not any(h.lower() == lower_value for h in headers):
+        headers.append(value)
+
+    updated_header = ', '.join(headers)
+    response.headers['Vary'] = updated_header
diff --git a/flask/sessions.py b/flask/sessions.py
index 2dbd8b32..47f0b3fd 100644
--- a/flask/sessions.py
+++ b/flask/sessions.py
@@ -9,18 +9,20 @@
     :license: BSD, see LICENSE for more details.
 """
 
-import uuid
 import hashlib
+import uuid
 import warnings
-from base64 import b64encode, b64decode
+from base64 import b64decode, b64encode
 from datetime import datetime
-from werkzeug.http import http_date, parse_date
+
+from itsdangerous import BadSignature, URLSafeTimedSerializer
 from werkzeug.datastructures import CallbackDict
+from werkzeug.http import http_date, parse_date
+
+from flask.helpers import patch_vary_header
 from . import Markup, json
 from ._compat import iteritems, text_type
-from .helpers import total_seconds, is_ip
-
-from itsdangerous import URLSafeTimedSerializer, BadSignature
+from .helpers import is_ip, total_seconds
 
 
 class SessionMixin(object):
@@ -405,7 +407,7 @@ class SecureCookieSessionInterface(SessionInterface):
 
         # Add a "Vary: Cookie" header if the session was accessed at all.
         if session.accessed:
-            response.headers.add('Vary', 'Cookie')
+            patch_vary_header(response, 'Cookie')
 
         if not self.should_set_cookie(app, session):
             return
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):