It’s a fundamental limitation of identical redundant systems that they have vulnerability to some of the same threats, particularly bad inputs and capacity issues. It’s important to understand it’s only giving you physical redundancy, such as if one data centre goes down. But the same software bugs, the same bad input data, even the same memory overruns are likely to hit both systems.

It’s not bad design, it’s just you have to understand what resiliency you have and plan against each of various such threats according to your risk appetite.