That there are two destroyers classes being produced at two shipyards is already sufficient redundancy in the absence of imminent conflict. And if more redundancy were deemed necessary it would be set up differently:Yes, but I think the military would've recognized the vulnerability of their shipyards in the event of war, and would've made a sensible and reasonable degree of redundancy planning without incurring excessive inefficiencies... including splitting production of its major destroyer classes between two shipyards.
Shipyard #1: 055, 052D
Shipyard #2: 055, 054AG
Shipyard #3 052D, 054AG
The other points you have raised (redundancy, upskilling) are well taken, however. As usual we will have to wait and see what eventuates.