A single check valve stops backflow, but miniscule amounts of "contaminated" water will have passed back along the pipe before it closes. In theory two single check valves in series anywhere along a pressure pipe will act jointly as a double check valve, but the farther they are apart the more likely it is that something will act to stop them doing so. Possibly the shower maker has found it easier to adapt a design by using single check valves either side of the mixer instead of a DVC elsewhere, perhaps on the grounds of length of a DCV fitting. Because of the typical design of both types of check valve they can restrict forward flow rates noticeably, and you should only have them if you really need them.