You don't want to use the Scandoubler FX option. That is a old and outdated video setting that doesn't work too well. It was a quick and basic solution that has since been superseded by the current filter features. They're found in the "system" menu when you have a core loaded under Video Processing. It should be one menu screen to the right of the starting menu page. There is where you'll find the current and much better system for scanlines, shadow masks, and etc that can be applied. It is a bit more complicated since it's so customizable but there is a "load preset" option that can help you get started. As it's a part of the main MiSTer system, it should be there regardless of if a core has kept up with recent developments. For example, Mappy is very far behind in features being over a year old but the Video Processing menu is there and functional. As long as you had update_all grab the presets and options for you, you should be good to experiment!
https://github.com/MiSTer-devel/Filters_MiSTer for the main page about them with a few examples. There's a lot you can do that goes deeper than just those basic examples, though. It's a topic worth looking up videos or examples people have posted.