Java 错误，重复的局部变量

I'm trying to debug a portion of code for an assignment (I'm still very new to Java) and have combed through many resources to solve this conflict but still can't quite work it out.

public static void main(String [] args){
    Scanner keyboard = new Scanner(System.in);
    String input = null;
    do
    {
      System.out.println("Enter 'A' for option A or 'B' for option B.");
      String input = keyboard.next();
      input.toLowerCase();
      input.charAt(0);  
    }
    while ((input != "a") || (input != "b"));
}

输入字符串总是出现重复的局部变量错误.

String input = keyboard.next();

input = keyboard.next();