Checks don't have to include the definition of knowledge independently, so multiple checks against the same rule don't need to be a violation. As a simple example, if you have a JSON Schema, that is the single source of truth for validation, and you can validate against it in 16 different places at different stages of processing and you haven't violated the principle that each piece of knowledge should be represented once in a system.

But yes, if you have truly separate codebases it becomes more difficult, and protocols need to be quite stable and changes to them carefully managed.

Or you derive one of the implementations from the other, or both from a third one.

But yeah, heterogeneous environments have a tendency of creating unnecessary code duplication.