The Oxford Style Guide says, for example, that:
When the final consonant is w, x, or y this is not doubled:
tow, towing, towed
vex, vexing, vexed
However, it seems that doxxed/doxxing are more common than doxed/doxing. (When I search on Google, doxxing has 4.82M results while doxing has only 1.48M.)
So which is "correct"? Double x's or single x?