Today, usually "duplex" means a load with two powders, arranged in layers. I also think the Army experimented with a "double" bullet in 7.62 NATO which they called duplex. Now, my memory may be playing tricks on me, but for some reason I associate Keith's use of the term to describe the use of a tube inside the case to carry the flash to the front of the powder charge. When testing .50 caliber BMG rounds with this, he got something like 202 ft/sec more velocity with normal pressure. Unfortunately, if fired in an oversize chamber, pressure "spiked."
Maybe I'm confusing that article with another I read about the same time...which must be at least a quarter century ago. Memory must be going . . .