Fixed double statement

allowed_contexts.dm_channel = True was stated twice, removed the second one.

discord/app_commands/commands.py

@@ -2628,7 +2628,6 @@ def dm_only(func: Optional[T] = None) -> Union[T, Callable[[T], T]]:
         allowed_contexts.private_channel = False  # Disable private channel context
         allowed_contexts.dm_channel = True  # Enable DM context
         
-        allowed_contexts.dm_channel = True
         return f
 
     # Check if called with parentheses or not