The Multicast will copy the source Exchange and multicast each copy. However the copy is a shallow copy, so in case you have mutateable message bodies, then any changes will be visible by the other copied messages. If you want to use a deep clone copy then you need to use a custom onPrepare which allows you to do this using the Processor interface.
Note that onPrepare can be used for any kind of custom logic which you would like to execute before the Exchange is being multicasted.
The Multicast EIP page on the Camel website hosts a dynamically updated example of using onPrepare to execute custom logic.