This sometimes falls under “be cautious with what you output, but generous (i.e. flexible) or very careful (full validation, good logging, making sure you fail safe upon receiving any/all unexpected input) with what you accept”. This usually makes duplication the worst choice because you could have to do a lot more thinking (and maybe coding) down the line to make sure all is well everywhere, and you need to document (or at least comment) so that others know these requirements when they make future changes, but it can be a valid approach especially in related but loosely coupled parts.