A very, very nesessary option for the Wishlist. I really miss it since VirtualDub is left behind with Windows itself.
I'll not explain how it worked in VD (one word: primitively!) and immediately depict it for kdenlive interface.
You select one video track as Master and when it's time to export the final cut, a "use Master compression" option appears among other compression/encoding options. That means more than just "use the same encoder and compression ratio as Master had": every part of every video which is fully compatible with Master data stream (in other words, has no effects applied, has the same encoder with exactly the same parameters, belongs to an unchanged/uncut part between two keyframes(!), has compatible frame rate and probably have to be in the same container format) is DIRECTLY copied from source video stream to target video stream without any re-encoding. That means, for simple copy-cut-paste operations on multiple similar files, only short intervals near keyframes are affected and require re-encoding, making the work nearly lossless. Of course, some containers and codecs don't allow that, but for ones which do, this option is incredibly powerful.
I really miss it, it was the best thing old VirtualDub had. Ever. If you're going to rework the file export routines, please try to support it at least partially, to a certain possible degree.