> Because that can't happen because of coherence. (Right?)

yes

Through you still can run into it when unsafe is involved, e.g. C FFI/no_mange or ASM with non-mangled labels as they are globally unique. Through IMHO, it's not a common problem and has ways to make it very unlikely for the projects where it matters.

In the end if you pull in C-FFI code (or provide it) you do ope yourself up to C ABI specific problems.