It’s in their confessions. If they disagree with their confessions that’s one thing but if they agree with their confessions by definition they believe in double predestination. Fudge words aside.
No, there are different confessions. If any particular confession is double predestinarian, then so be it.
Personally, I don’t see how, in terms of its relevance for the human beings involved, that it is significant. If you’re not one of those picked to be the saved, then you’re one of the damned, whether specifically picked to be so or not. If you are picked to be one of the damned, then you’re still one of the damned.
Either way, damned if you do and damned if you don’t. :>)