How about render the frame just once and duplicate it post using Relove or AfterEffects?
That's a workaround, but forces user to either cut up their animation renders to account for that, or change their animations to account for it prior to queueing,which could mess up timings and such depending on the need/use of the animation. But then it also adds post processing time to search out those frames, cut the sequence, then either duplicate or modify time settings to fill the space needed with that frame. Again, time consuming - possibly equal to or more than the extra time of rendering the duplicate frames.
There has to be a way to add some intelligence to the code for rendering where it analyzes the frames and finds areas where there are duplicate frames and adjusts itself accordingly and duplicating frames as a native act. I would hope so at least.