Why re-declaration of global variables is valid but not local of local variables?

Question

If I declare i to be a global variable twice, then it works fine:

#include <stdio.h>
int i;int i;
int main(void) 
{
    printf("%d",i);
    return 0;
}

But not if I declare it locally (twice)

#include <stdio.h>
int main(void) 
{
    int i;int i;
printf("%d",i);
return 0;
}

Error

error: redeclaration of ‘i’ with no linkage

Also, if in the 1st case I initialize i to some value i=1, then also it gives error error: redefinition of ‘i’

Why is does it exhibit such a behaviour ?

Thanks!