diff --git a/fastapi/openapi/utils.py b/fastapi/openapi/utils.py
index 1c7a17c4ca..21b536766b 100644
--- a/fastapi/openapi/utils.py
+++ b/fastapi/openapi/utils.py
@@ -82,24 +82,23 @@ def get_openapi_security_definitions(
     flat_dependant: Dependant,
 ) -> tuple[dict[str, Any], list[dict[str, Any]]]:
     security_definitions = {}
-    # Use a dict to merge scopes for same security scheme
-    operation_security_dict: dict[str, list[str]] = {}
+    # Use a dict of sets to merge scopes for same security scheme
+    operation_security_dict: dict[str, set[str]] = {}
     for security_dependency in flat_dependant._security_dependencies:
-        security_definition = jsonable_encoder(
-            security_dependency._security_scheme.model,
-            by_alias=True,
-            exclude_none=True,
-        )
         security_name = security_dependency._security_scheme.scheme_name
-        security_definitions[security_name] = security_definition
-        # Merge scopes for the same security scheme
+        if security_name not in security_definitions:
+            security_definition = jsonable_encoder(
+                security_dependency._security_scheme.model,
+                by_alias=True,
+                exclude_none=True,
+            )
+            security_definitions[security_name] = security_definition
         if security_name not in operation_security_dict:
-            operation_security_dict[security_name] = []
+            operation_security_dict[security_name] = set()
         for scope in security_dependency.oauth_scopes or []:
-            if scope not in operation_security_dict[security_name]:
-                operation_security_dict[security_name].append(scope)
+            operation_security_dict[security_name].add(scope)
     operation_security = [
-        {name: scopes} for name, scopes in operation_security_dict.items()
+        {name: sorted(scopes)} for name, scopes in operation_security_dict.items()
     ]
     return security_definitions, operation_security