Sample code snippet

const const const int x = 10;   
int main()
{}

gets compiled in C but not in C++. Why does it get compiled in C? I thought this would fail in C as well. Never mind.

Which part of the C++ Standard forbids the use of duplicate const and which part of the C standard allows this?

C99 §6.7.3/4:

If the same qualifier appears more than once in the same specifier-qualifier-list, either directly or via one or more typedef s, the behavior is the same as if it appeared only once.

Yes, that is valid C99, and your discovery is correct.