I'm not clear whether you even need to try to improve things on that machine or are running at maximum speed.
If the specification of your laser printer is rated at a simplex print speed of 50ppm (and looking the HP9050 up it is) and you are getting 25-26ppm in duplex mode.. then you are probably benefiting from a pretty efficient duplex print path and fast rendering.. because you are on average getting the result of single sheet printing twice per page.
It could be much worse than this because many printers historically took a significant printing throughput hit because of the design of their duplexing mechanism effectively stalling the top simplex speed - meaning you might be better off putting the same stack through simplex twice.
Alternatively double-sided throughput might be better because some better designs seem to get a higher throughput duplex speed than half the simplex speed - probably because the sheet approaching for second image printing may internally be significantly closer in the internal paper path than a fresh sheet being lifted from a paper tray. A super efficient mechanism might benefit by a pipeline where the machine is lifting the next blank sheet (in readiness for its first side printing) while an earlier half printed sheet is heading for it's second image. If the pipeline of activity is deep enough on long enough jobs then you can see the machine could approach duplex speeds which are equal to top simplex speeds. I think that may be what you are expecting in this instance, but I think you may be disappointed.
More complex paper paths probably leads to more engineering problems in design and potential impact of jams (including confusion with restarts because a jammed page may precede a successful print..) which means not all printer companies seek to put them into machines. Large and multiple paper reservoirs means that some paper drawers may print at marginally different throughput speeds than others. Run some experiments with blank pages and work out what the optimal speed is for your system.. I've certainly experienced (multi-hopper folding) machines where I choose to live refill a small amount in one hopper and max out another to improve large job throughput.
Really, it would do us all a favour if all laser printer companies would advertise native duplex machines with their average duplex page speeds based on 50sheet (ie. 100 sides) duplex loads.. but they don't.