It is not redundant, it makes a copy of the file for the parallel test below.
See concurrency.

Note there is a long standing issue to detect the type of error that can happen if this copy was not made. #16520

I assume also the race detector may complain if the copy was not there.

Thanks, I have gained knowledge!

I write some code to test this issue:

package main

import "fmt"

func main() {
        a := []int{1,2,3}
        for x := range a {
                fmt.Println(x)
                fmt.Println(&x)
                x := x
                fmt.Println(&x)
        }
}

test logs:

$ go run main.go 
0
0xc00007e010
0xc00007e030
1
0xc00007e010
0xc00007e040
2
0xc00007e010
0xc00007e050