compiler: fix check for duplicate declaration

compiler: fix check for duplicate declaration

    The compiler check that issued a duplicate declaration error for
        a, a, a := 1, 2, 3
    was incorrectly issuing an error for
        a, a, a = 1, 2, 3
    While this is not particularly useful, it is valid Go.

    Test is https://golang.org/cl/25143.

    Reviewed-on: https://go-review.googlesource.com/25144

From-SVN: r238618