diff --git a/src/click/types.py b/src/click/types.py
index a735fbba3..c73c1a3b6 100644
--- a/src/click/types.py
+++ b/src/click/types.py
@@ -253,9 +253,10 @@ def to_info_dict(self) -> t.Dict[str, t.Any]:
 
     def get_metavar(self, param: "Parameter") -> str:
         if param.param_type_name == "option" and not param.show_choices:
-            choices_str = "|".join(
-                {convert_type(type(choice)).name.upper() for choice in self.choices}
-            )
+            choice_metavars = [
+                convert_type(type(choice)).name.upper() for choice in self.choices
+            ]
+            choices_str = "|".join([*dict.fromkeys(choice_metavars)])
         else:
             choices_str = "|".join([str(i) for i in self.choices])