From f7c318c8197a08d12a408113565618f462436a4c Mon Sep 17 00:00:00 2001
From: Peter Aradi <aradipe@gmail.com>
Date: Fri, 28 Oct 2022 12:26:43 +0200
Subject: [PATCH 1/2] Do not allow duplicate tag names (OpenAPI object)

Addresses issue #5552
---
 fastapi/openapi/models.py            | 10 +++++++++-
 fastapi/openapi/utils.py             |  5 ++++-
 tests/test_duplicate_openapi_tags.py | 22 ++++++++++++++++++++++
 3 files changed, 35 insertions(+), 2 deletions(-)
 create mode 100644 tests/test_duplicate_openapi_tags.py

diff --git a/fastapi/openapi/models.py b/fastapi/openapi/models.py
index 35aa1672b..40f8aee78 100644
--- a/fastapi/openapi/models.py
+++ b/fastapi/openapi/models.py
@@ -2,7 +2,7 @@ from enum import Enum
 from typing import Any, Callable, Dict, Iterable, List, Optional, Union
 
 from fastapi.logger import logger
-from pydantic import AnyUrl, BaseModel, Field
+from pydantic import AnyUrl, BaseModel, Field, validator
 
 try:
     import email_validator  # type: ignore
@@ -400,6 +400,14 @@ class OpenAPI(BaseModel):
     class Config:
         extra = "allow"
 
+    @validator("tags")
+    def check_tags(cls, tags):  # type: ignore
+        unique_names = set()
+        assert not any(
+            t.name in unique_names or unique_names.add(t.name) for t in tags  # type: ignore
+        ), "Tag names must be unique"
+        return tags
+
 
 Schema.update_forward_refs()
 Operation.update_forward_refs()
diff --git a/fastapi/openapi/utils.py b/fastapi/openapi/utils.py
index 86e15b46d..9fbc8fdff 100644
--- a/fastapi/openapi/utils.py
+++ b/fastapi/openapi/utils.py
@@ -444,5 +444,8 @@ def get_openapi(
         output["components"] = components
     output["paths"] = paths
     if tags:
-        output["tags"] = tags
+        # discard tags with non-unique names as it is against the OpenAPI spec
+        # https://swagger.io/specification/#openapi-object
+        names = set()
+        output["tags"] = [t for t in tags if t["name"] not in names and not names.add(t["name"])]  # type: ignore
     return jsonable_encoder(OpenAPI(**output), by_alias=True, exclude_none=True)  # type: ignore
diff --git a/tests/test_duplicate_openapi_tags.py b/tests/test_duplicate_openapi_tags.py
new file mode 100644
index 000000000..584d21275
--- /dev/null
+++ b/tests/test_duplicate_openapi_tags.py
@@ -0,0 +1,22 @@
+"""Test case for possible tag duplication at OpenAPI object level"""
+
+from fastapi import FastAPI
+from fastapi.testclient import TestClient
+
+app = FastAPI(
+    openapi_tags=[
+        {"name": "items", "description": "items1"},
+        {"name": "items", "description": "items2"},
+    ]
+)
+
+client = TestClient(app)
+
+
+def test_openapi_for_duplicates():
+    response = client.get("/openapi.json")
+    assert response.status_code == 200, response.text
+    tag_list = response.json()["tags"]
+    assert len(tag_list) == 1
+    assert tag_list[0]["name"] == "items"
+    assert tag_list[0]["description"] == "items1"

From fc8a4e11fc4b0351b82374f6603601053a7041ce Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Thu, 12 Jun 2025 12:58:48 +0000
Subject: [PATCH 2/2] =?UTF-8?q?=F0=9F=8E=A8=20[pre-commit.ci]=20Auto=20for?=
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---
 fastapi/openapi/models.py | 4 ++--
 fastapi/openapi/utils.py  | 4 +++-
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/fastapi/openapi/models.py b/fastapi/openapi/models.py
index d5eee02d4..e2abca215 100644
--- a/fastapi/openapi/models.py
+++ b/fastapi/openapi/models.py
@@ -14,7 +14,6 @@ from pydantic import AnyUrl, BaseModel, Field, validator
 from typing_extensions import Annotated, Literal, TypedDict
 from typing_extensions import deprecated as typing_deprecated
 
-
 try:
     import email_validator
 
@@ -444,7 +443,8 @@ class OpenAPI(BaseModelWithConfig):
     def check_tags(cls, tags):  # type: ignore
         unique_names = set()
         assert not any(
-            t.name in unique_names or unique_names.add(t.name) for t in tags  # type: ignore
+            t.name in unique_names or unique_names.add(t.name)
+            for t in tags  # type: ignore
         ), "Tag names must be unique"
         return tags
 
diff --git a/fastapi/openapi/utils.py b/fastapi/openapi/utils.py
index e51cafb56..5f533b8c2 100644
--- a/fastapi/openapi/utils.py
+++ b/fastapi/openapi/utils.py
@@ -568,5 +568,7 @@ def get_openapi(
         # discard tags with non-unique names as it is against the OpenAPI spec
         # https://swagger.io/specification/#openapi-object
         names = set()
-        output["tags"] = [t for t in tags if t["name"] not in names and not names.add(t["name"])]  # type: ignore
+        output["tags"] = [
+            t for t in tags if t["name"] not in names and not names.add(t["name"])
+        ]  # type: ignore
     return jsonable_encoder(OpenAPI(**output), by_alias=True, exclude_none=True)  # type: ignore