When running JFrame Code I get this error?

I've used a jFrame and ever since I only adjusted this small part of code...

private void RandomNumberActionPerformed(java.awt.event.ActionEvent evt) {                                             

    Random rn = new Random();
    RandomNumber.setText(Integer.toString(rn.nextInt(51)));  

and since I've been getting these errors.

Exception in thread "AWT-EventQueue-0" java.lang.ClassFormatError: Duplicate field name&signature in class file compsciia/NumberSide
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:760)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:455)
at java.net.URLClassLoader.access$100(URLClassLoader.java:73)
at java.net.URLClassLoader$1.run(URLClassLoader.java:367)
at java.net.URLClassLoader$1.run(URLClassLoader.java:361)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:360)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
at compsciia.MainMenu.StartActionPerformed(MainMenu.java:91)
at compsciia.MainMenu.access$000(MainMenu.java:13)
at compsciia.MainMenu$1.actionPerformed(MainMenu.java:44)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2346)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259) **etc**

It was working perfectly fine before. I'm really not sure what to do? I have two classes, my main menu class which opens up this class (the one with the error).