A duplex load is one where two powders are loaded into the case, generally a slow powder is loaded first, with a faster powder on top (at least that is how I remember Ackley writing about it). The theory is that the slow powder will push the bullet down the bore a bit and expand the effective case capacity for combustion when the fast powder kicks in and keeps pressures high for a bit longer. This method was designed to get past the "performance plateau" of "more isn't always better" when it comes to powder charges (sometimes adding powder does not add to velocity in a linear pattern, sometimes it tapers off or "plateaus").
With an ultra slow powder like 872, some folks are going the opposite route and using a fast powder to augment the primer in building initial pressure. I haven't found any "hard and fast" rules with how they do this, but reports go anywhere from a few grains, to 5% of the total charge weight. So it ends up fast powder on the bottom, slow powder on top.
In theory, a compressed load will prevent the powders from mixing due to normal handling.
Machine guns are awesome until you have to carry one.