Why can't I declare a variable with the same name that has already been declared, but the new variable is beyond the scope of another variable

private void Abc()
{
    string first="";
    ArrayList result = new ArrayList();
    ArrayList secResult = new ArrayList();
    foreach (string second in result)
    {
        if (first != null)
        {
            foreach (string third in secResult)
            {
                string target;
            }
        }

        string target;//Here I cannot decalre it. And if I don't declare it and
        //want to use it then also I cannot use it. And if it is in if condition like
        //the code commented below, then there is no any complier error.
        //if (first != null)
        //{
        //    string target;
        //}
    }
}

I cannot understand: why I cannot declare a variable outside the loop foreach, since the compiler gives an error that the variable is already declared. The scope of the variable foreach(and therefore target) is completed, where I declare this new variable.