Pressure, most smokeless don't take kindly to compression. You've already dumped a 1/3 or more of the smokeless maximum charge in the case, and then finish filling the case with black. Which the black by it's own self would be running very nearly maximum pressure.
Duplexing is an interesting concept, but with no real practical application anymore. NRA rules prohibit any smokeless in bpcr/bptr matches, and if attending a gong match, and don't want to shoot the black, then just shoot smokeless.. Cause even with duplexing you still have to clean the gun and the cases.
Besides all that there have been some that swore that shooting duplex was a sure indication of that persons only prom date being their mom's baby sister....
