I seem to have had the same problem - Caesar and Brennus were already at war. Caesar asked me to declare war on Brennus. I decided to, and so selected the appropriate button. But instead of declaring war only on Brennus, the computer announced I had declared war on BOTH Brennus and Caesar! I definitely didn't want to fight Caesar, and hadn't saved the game to go back and reload. Very frustrating, and appears to me to be a bug.