What does it occur when a global variable is defined twice in the same scope?

Question

For example, vrb is a global variable:

<script>
    var vrb = 25

    if (true) {
        var vrb = 15
    }

    document.write(vrb)
</script>

, what did it occured in my script related to vrb variable? I know about shadowing but I think that this is not the term for what I asked.