How to fix "variable previously declared 'static' redeclared 'extern' error"?

When I try:

#include <stdio.h>

static int c1 = 10;

int main(){

    {
        extern int c1; //c1 here still has internal linkage
        printf("%d",c1);
    }

return 0;
}

Nothing goes wrong. But when I try to modify it a little bit like this:

#include <stdio.h>

static int c1 = 10;

int main(){

    int c1 = 20; //This is where I modified.
    c1++;

    {
        extern int c1;
        printf("%d",c1);
    }

return 0;
}

And suddenly, there's an error coming out. It says "variable previously declared 'static' redeclared 'extern'". Why?

Verbatim error message:

prog.c: In function ‘main’:
prog.c:11:20: error: variable previously declared ‘static’ redeclared ‘extern’
         extern int c1;