Seems pretty basic to me that if you double something then you increase it or raise it UP, viz the English expression "doubling up"
"Double down" makes no sense IMO and only seems to have appeared recently. Presumably it's an Americanism that's trying to ooze its way in over here.
It should be placed in the 'Rejected' bin, along with 'Gotten', 'Nite Club', 'Donut', etc. - oh, and 'Herb', when pronounced (in English) without the 'H' .
'arry 'oudini sat in the 'erb garden - Yes or No?
(Cockneys kindly ignore)