# UNDF: UNDF-2026-000000069
--- a/fastapi/dependencies/utils.py
+++ b/fastapi/dependencies/utils.py
@@ -139,13 +139,13 @@ def _get_dependant_for_depends(
 def get_flat_dependant(
     dependant: Dependant,
     *,
     skip_repeats: bool = False,
-    visited: list[DependencyCacheKey] | None = None,
+    visited: set[DependencyCacheKey] | None = None,
     parent_oauth_scopes: list[str] | None = None,
 ) -> Dependant:
     if visited is None:
-        visited = []
-    visited.append(dependant.cache_key)
+        visited = set()
+    visited.add(dependant.cache_key)
     use_parent_oauth_scopes = (parent_oauth_scopes or []) + (
         dependant.oauth_scopes or []
     )