importC: symbol duplication in symbol table for global redeclared symbols

in foo.c
```   int a;
    int a;
    int a;
    int a;
    int a;
    int a;
    int a;
```

`dmd -c foo.c`
`nm foo.o`

```
0000000000000000 B _a
0000000000000004 B _a
0000000000000008 B _a
000000000000000c B _a
0000000000000010 B _a
0000000000000014 B _a
```


This is a minor issue that needs to be fixed. generate only one symbol for a in the BSS which is what happens with the C compilers.


