Where regular nigger connotes simplistic worthlessness, double nigger adds an all but unmatchable level of contempt and disdain for the despicably niggery act.
A similar Robotnik meme also reveals this to be fungible, with the line:
300 niggers? That's as many as 150 double niggers.
i_scream_trucks 1 points 4 months ago
how long is a piece of nigger?
twice half its nigger, or half double its nigger.
/nods firmly