The problem with generating ODR violation diagnostics is that it requires the linker to compare all the duplicate definitions to each other not on bit-by-bit basis, but somehow discerning whether they are generated from same input source or functionally equivalent, without embedding some hairy metadata in the object file this seems like something that reduces to halting problem.

This is also one of the many problems that simply ceases to exists once you move away from the C/C++ model of "preprocessor as module system".