Diodes Dx are connected in parallel to the MOSFET internal substrate diodes and not generally required. Bus voltage bypass capacitors are however strongly recommended.
In some cases, fast free-wheeling diodes are added to the bridge output stage. This mainly works with schottky diodes, that have a lower forward voltage than the substrate diodes. But preferably, a H-bridge would be controlled to achieve synchronous switching, so the output current is never carried by a diode.