diff --git a/httpx/_models.py b/httpx/_models.py
index f6cd6db939..3755c2548c 100644
--- a/httpx/_models.py
+++ b/httpx/_models.py
@@ -964,8 +964,10 @@ def get_list(self, key: str, split_commas: bool = False) -> typing.List[str]:
 
     def update(self, headers: HeaderTypes = None) -> None:  # type: ignore
         headers = Headers(headers)
-        for key, value in headers.raw:
-            self[key.decode(headers.encoding)] = value.decode(headers.encoding)
+        for key in headers.keys():
+            if key in self:
+                self.pop(key)
+        self._list.extend(headers._list)
 
     def copy(self) -> "Headers":
         return Headers(self, encoding=self.encoding)
diff --git a/tests/client/test_headers.py b/tests/client/test_headers.py
index ba862fd521..264ca0bd67 100755
--- a/tests/client/test_headers.py
+++ b/tests/client/test_headers.py
@@ -10,6 +10,16 @@ def echo_headers(request: httpx.Request) -> httpx.Response:
     return httpx.Response(200, json=data)
 
 
+def echo_repeated_headers_multi_items(request: httpx.Request) -> httpx.Response:
+    data = {"headers": list(request.headers.multi_items())}
+    return httpx.Response(200, json=data)
+
+
+def echo_repeated_headers_items(request: httpx.Request) -> httpx.Response:
+    data = {"headers": list(request.headers.items())}
+    return httpx.Response(200, json=data)
+
+
 def test_client_header():
     """
     Set a header in the Client.
@@ -110,6 +120,35 @@ def test_header_update():
     }
 
 
+def test_header_repeated_items():
+    url = "http://example.org/echo_headers"
+    client = httpx.Client(transport=httpx.MockTransport(echo_repeated_headers_items))
+    response = client.get(url, headers=[("x-header", "1"), ("x-header", "2,3")])
+
+    assert response.status_code == 200
+
+    echoed_headers = response.json()["headers"]
+    # as per RFC 7230, the whitespace after a comma is insignificant
+    # so we split and strip here so that we can do a safe comparison
+    assert ["x-header", ["1", "2", "3"]] in [
+        [k, [subv.lstrip() for subv in v.split(",")]] for k, v in echoed_headers
+    ]
+
+
+def test_header_repeated_multi_items():
+    url = "http://example.org/echo_headers"
+    client = httpx.Client(
+        transport=httpx.MockTransport(echo_repeated_headers_multi_items)
+    )
+    response = client.get(url, headers=[("x-header", "1"), ("x-header", "2,3")])
+
+    assert response.status_code == 200
+
+    echoed_headers = response.json()["headers"]
+    assert ["x-header", "1"] in echoed_headers
+    assert ["x-header", "2,3"] in echoed_headers
+
+
 def test_remove_default_header():
     """
     Remove a default header from the Client.