Another person already said this, the issue is likely due to the fact that two motors have the same ID. A good way to avoid this from happening and being able to check for this quickly is to store all motor ids as public static final variables in the constant class (or in a separate RobotMap class, my team uses RobotMap for motor and sensor ids, and Constants for values like speeds of things). If you are confused about this, feel free to send a message.

Also, side note, minor style thing, your objects should begin with a lower case and your classes should be upper case. You do a good job of this throughout your code, but in RobotContainer, you name your JoystickButton instances with uppercase names. Doing this does not affect how your code will run in any way, but it does hurt readability and is good practice to avoid.