Do not allow duplicate tag names (OpenAPI object)

Addresses issue #5552

fastapi/openapi/models.py

@@ -2,7 +2,7 @@
 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_foo(cls, v):  # type: ignore
+        unique_names = set()
+        assert not any(
+            t.name in unique_names or unique_names.add(t.name) for t in v  # type: ignore
+        ), "Tag names must be unique"
+        return v
+
 
 Schema.update_forward_refs()
 Operation.update_forward_refs()

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

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"