diff --git a/tests/proxy_unit_tests/test_proxy_utils.py b/tests/proxy_unit_tests/test_proxy_utils.py
index 607e54225..77e78621d 100644
--- a/tests/proxy_unit_tests/test_proxy_utils.py
+++ b/tests/proxy_unit_tests/test_proxy_utils.py
@@ -574,3 +574,108 @@ def test_get_docs_url(env_vars, expected_url):
 
     result = _get_docs_url()
     assert result == expected_url
+
+
+@pytest.mark.parametrize(
+    "request_tags, tags_to_add, expected_tags",
+    [
+        (None, None, []),  # both None
+        (["tag1", "tag2"], None, ["tag1", "tag2"]),  # tags_to_add is None
+        (None, ["tag3", "tag4"], ["tag3", "tag4"]),  # request_tags is None
+        (
+            ["tag1", "tag2"],
+            ["tag3", "tag4"],
+            ["tag1", "tag2", "tag3", "tag4"],
+        ),  # both have unique tags
+        (
+            ["tag1", "tag2"],
+            ["tag2", "tag3"],
+            ["tag1", "tag2", "tag3"],
+        ),  # overlapping tags
+        ([], [], []),  # both empty lists
+        ("not_a_list", ["tag1"], ["tag1"]),  # request_tags invalid type
+        (["tag1"], "not_a_list", ["tag1"]),  # tags_to_add invalid type
+        (
+            ["tag1", "tag1"],
+            ["tag1", "tag2"],
+            ["tag1", "tag2"],
+        ),  # duplicate tags in inputs
+    ],
+)
+def test_merge_tags(request_tags, tags_to_add, expected_tags):
+    from litellm.proxy.litellm_pre_call_utils import LiteLLMProxyRequestSetup
+
+    result = LiteLLMProxyRequestSetup._merge_tags(
+        request_tags=request_tags, tags_to_add=tags_to_add
+    )
+
+    assert isinstance(result, list)
+    assert sorted(result) == sorted(expected_tags)
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize(
+    "key_tags, request_tags, expected_tags",
+    [
+        # exact duplicates
+        (["tag1", "tag2", "tag3"], ["tag1", "tag2", "tag3"], ["tag1", "tag2", "tag3"]),
+        # partial duplicates
+        (
+            ["tag1", "tag2", "tag3"],
+            ["tag2", "tag3", "tag4"],
+            ["tag1", "tag2", "tag3", "tag4"],
+        ),
+        # duplicates within key tags
+        (["tag1", "tag1", "tag2"], ["tag3", "tag4"], ["tag1", "tag2", "tag3", "tag4"]),
+        # duplicates within request tags
+        (["tag1", "tag2"], ["tag3", "tag3", "tag4"], ["tag1", "tag2", "tag3", "tag4"]),
+        # case sensitive duplicates
+        (["Tag1", "TAG2"], ["tag1", "tag2"], ["Tag1", "TAG2", "tag1", "tag2"]),
+    ],
+)
+async def test_add_litellm_data_to_request_duplicate_tags(
+    key_tags, request_tags, expected_tags
+):
+    """
+    Test to verify duplicate tags between request and key metadata are handled correctly
+
+
+    Aggregation logic when checking spend can be impacted if duplicate tags are not handled correctly.
+
+    User feedback:
+    "If I register my key with tag1 and
+    also pass the same tag1 when using the key
+    then I see tag1 twice in the
+    LiteLLM_SpendLogs table request_tags column. This can mess up aggregation logic"
+    """
+    mock_request = Mock(spec=Request)
+    mock_request.url.path = "/chat/completions"
+    mock_request.query_params = {}
+    mock_request.headers = {}
+
+    # Setup key with tags in metadata
+    user_api_key_dict = UserAPIKeyAuth(
+        api_key="test_api_key",
+        user_id="test_user_id",
+        org_id="test_org_id",
+        metadata={"tags": key_tags},
+    )
+
+    # Setup request data with tags
+    data = {"metadata": {"tags": request_tags}}
+
+    # Process request
+    proxy_config = Mock()
+    result = await add_litellm_data_to_request(
+        data=data,
+        request=mock_request,
+        user_api_key_dict=user_api_key_dict,
+        proxy_config=proxy_config,
+    )
+
+    # Verify results
+    assert "metadata" in result
+    assert "tags" in result["metadata"]
+    assert sorted(result["metadata"]["tags"]) == sorted(
+        expected_tags
+    ), f"Expected {expected_tags}, got {result['metadata']['tags']}"