执行groovy脚本时出错

class Book
{
     private String title
     Book (String theTitle)
     {
         title=theTitle
     }
     String getTitle()
     {
         return title
     }
}
Book gina=new Book('Groovy in Action')
assert gina.getTitle()=='Groovy in Action'
assert getTitleBackwards(gina)=='noitcA ni yvoorG'
String getTitleBackwards(Book)
{
    title=book.getTitle()
    return title.reverse()
}

当我用Ctrl R执行时,我得到以下编译错误.

1编译错误：

Invalid duplicate class definition of class Book : The source
Book.groovy contains at least two definitions of the class Book. One
of the classes is an explicit generated class using the class
statement,the other is a class generated from the script body based
on the file name. Solutions are to change the file name or to change
the class name. at line: 1,column: 1

有人可以解释一下这里发生了什么.