What's The Purpose of One Additional Diode Here
This will improve how the circuit handles reverse voltage spikes as it may be seen by the designer (a bad design is not excluded).
It will not, however, simply double the maximum reverse voltage - it's a lot more complicated than that.
It will also allow the circuit to continue working in case one of the diodes fails as a short - this might be a common enough failure mode. (as OP I don't count on the probability of such event but I respect the editor)
Edited by @FrancoVS