import java.util.*;
public class test  {
    public static void main(String []args){
        
        
        ArrayList<String> list = new ArrayList<String>();
        list.add("how");
        list.add("are");
        list.add("you");
        
        
        
        doubleList(list);
        System.out.print(list.toString());
        
    }
    public static void doubleList(ArrayList<String> list){
        for(int i = 0; i<list.size(); i++){
            list.add(i*2,list.get(i*2) );
            
        }
        
    }
    
    
}

/**
     * Checks if the given index is in range.  If not, throws an appropriate
     * runtime exception.  This method does *not* check if the index is
     * negative: It is always used immediately prior to an array access,
     * which throws an ArrayIndexOutOfBoundsException if index is negative.
     */
    private void RangeCheck(int index) {
    if (index >= size)
        throw new IndexOutOfBoundsException(
        "Index: "+index+", Size: "+size);
    }

------解决方案--------------------
 for(int i = 0; i<list.size(); i++){
      list.add(i*2,list.get(i*2) );

原排列 0 1 2 
要加到 3 4 5 位

可程序中写成加到 0 2 4(注意  list.get(4)时候溢出。)
int size = list.size();     //计算原list大小。
使用list.add(i+size,list.get(i));

------解决方案--------------------
public static void doubleList(ArrayList<String> list){
       for(int i = 0; i<list.size(); i++){
           list.add(i,list.get(i) );
           list.add(i+1,list.get(i) );
       }
      
   }
这样不行吗？

------解决方案--------------------
这是根据楼主的意思实现的代码，请参考
Java code

import java.util.*;
public class Test{
    public static void main(String []args){


        ArrayList<String> list = new ArrayList<String>();
        list.add("how");
        list.add("are");
        list.add("you");


        doubleList(list);
        System.out.print(list.toString());

    }
    public static void doubleList(ArrayList<String> list){
        String[] strAry=new String[list.size()];
        list.toArray(strAry);
//        for(String s:strAry){
//            System.out.println(s);
//        }
        for(int i = 0; i<strAry.length; i++){
            list.add(i*2,strAry[i] );

        }

    }


}

------解决方案--------------------