diff --git a/src/openai/_base_client.py b/src/openai/_base_client.py
index 7a1562461..d7e5127dd 100644
--- a/src/openai/_base_client.py
+++ b/src/openai/_base_client.py
@@ -61,7 +61,7 @@
     RequestOptions,
     ModelBuilderProtocol,
 )
-from ._utils import is_dict, is_given, is_mapping
+from ._utils import is_dict, is_list, is_given, is_mapping
 from ._compat import model_copy, model_dump
 from ._models import GenericModel, FinalRequestOptions, validate_type, construct_type
 from ._response import (
@@ -451,14 +451,18 @@ def _build_request(
 
         headers = self._build_headers(options)
         params = _merge_mappings(self._custom_query, options.params)
+        content_type = headers.get("Content-Type")
 
         # If the given Content-Type header is multipart/form-data then it
         # has to be removed so that httpx can generate the header with
         # additional information for us as it has to be in this form
         # for the server to be able to correctly parse the request:
         # multipart/form-data; boundary=---abc--
-        if headers.get("Content-Type") == "multipart/form-data":
-            headers.pop("Content-Type")
+        if content_type is not None and content_type.startswith("multipart/form-data"):
+            if "boundary" not in content_type:
+                # only remove the header if the boundary hasn't been explicitly set
+                # as the caller doesn't want httpx to come up with their own boundary
+                headers.pop("Content-Type")
 
             # As we are now sending multipart/form-data instead of application/json
             # we need to tell httpx to use it, https://www.python-httpx.org/advanced/#multipart-file-encoding
@@ -494,9 +498,25 @@ def _serialize_multipartform(self, data: Mapping[object, object]) -> dict[str, o
         )
         serialized: dict[str, object] = {}
         for key, value in items:
-            if key in serialized:
-                raise ValueError(f"Duplicate key encountered: {key}; This behaviour is not supported")
-            serialized[key] = value
+            existing = serialized.get(key)
+
+            if not existing:
+                serialized[key] = value
+                continue
+
+            # If a value has already been set for this key then that
+            # means we're sending data like `array[]=[1, 2, 3]` and we
+            # need to tell httpx that we want to send multiple values with
+            # the same key which is done by using a list or a tuple.
+            #
+            # Note: 2d arrays should never result in the same key at both
+            # levels so it's safe to assume that if the value is a list,
+            # it was because we changed it to be a list.
+            if is_list(existing):
+                existing.append(value)
+            else:
+                serialized[key] = [existing, value]
+
         return serialized
 
     def _maybe_override_cast_to(self, cast_to: type[ResponseT], options: FinalRequestOptions) -> type[ResponseT]:
diff --git a/tests/test_client.py b/tests/test_client.py
index 3d2dd3582..24933456b 100644
--- a/tests/test_client.py
+++ b/tests/test_client.py
@@ -437,6 +437,35 @@ def test_request_extra_query(self) -> None:
         params = dict(request.url.params)
         assert params == {"foo": "2"}
 
+    def test_multipart_repeating_array(self, client: OpenAI) -> None:
+        request = client._build_request(
+            FinalRequestOptions.construct(
+                method="get",
+                url="/foo",
+                headers={"Content-Type": "multipart/form-data; boundary=6b7ba517decee4a450543ea6ae821c82"},
+                json_data={"array": ["foo", "bar"]},
+                files=[("foo.txt", b"hello world")],
+            )
+        )
+
+        assert request.read().split(b"\r\n") == [
+            b"--6b7ba517decee4a450543ea6ae821c82",
+            b'Content-Disposition: form-data; name="array[]"',
+            b"",
+            b"foo",
+            b"--6b7ba517decee4a450543ea6ae821c82",
+            b'Content-Disposition: form-data; name="array[]"',
+            b"",
+            b"bar",
+            b"--6b7ba517decee4a450543ea6ae821c82",
+            b'Content-Disposition: form-data; name="foo.txt"; filename="upload"',
+            b"Content-Type: application/octet-stream",
+            b"",
+            b"hello world",
+            b"--6b7ba517decee4a450543ea6ae821c82--",
+            b"",
+        ]
+
     @pytest.mark.respx(base_url=base_url)
     def test_basic_union_response(self, respx_mock: MockRouter) -> None:
         class Model1(BaseModel):
@@ -1104,6 +1133,35 @@ def test_request_extra_query(self) -> None:
         params = dict(request.url.params)
         assert params == {"foo": "2"}
 
+    def test_multipart_repeating_array(self, async_client: AsyncOpenAI) -> None:
+        request = async_client._build_request(
+            FinalRequestOptions.construct(
+                method="get",
+                url="/foo",
+                headers={"Content-Type": "multipart/form-data; boundary=6b7ba517decee4a450543ea6ae821c82"},
+                json_data={"array": ["foo", "bar"]},
+                files=[("foo.txt", b"hello world")],
+            )
+        )
+
+        assert request.read().split(b"\r\n") == [
+            b"--6b7ba517decee4a450543ea6ae821c82",
+            b'Content-Disposition: form-data; name="array[]"',
+            b"",
+            b"foo",
+            b"--6b7ba517decee4a450543ea6ae821c82",
+            b'Content-Disposition: form-data; name="array[]"',
+            b"",
+            b"bar",
+            b"--6b7ba517decee4a450543ea6ae821c82",
+            b'Content-Disposition: form-data; name="foo.txt"; filename="upload"',
+            b"Content-Type: application/octet-stream",
+            b"",
+            b"hello world",
+            b"--6b7ba517decee4a450543ea6ae821c82--",
+            b"",
+        ]
+
     @pytest.mark.respx(base_url=base_url)
     async def test_basic_union_response(self, respx_mock: MockRouter) -> None:
         class Model1(BaseModel):