issue about `go range`

func main() {
    f := map[string]interface{}{
        "a": "a",
    }

    for _, v := range f {
        v := v.(string)
    }

    w := f["a"]
    w := w.(string)
}

// .\test.go:13:7: no new variables on left side of :=

range 迭代时v := v.(string)scope 里已经存在一个 v 变量，为什么可以定义同名新变量呢