Double circulation system is usually found in complex organisms like us , humans.It has two parts - Pulmonary and Systemic.
Let's start from pulmonary circulation . The right ventricle pumps the deoxygenated blood into the lungs and it is returned in the left auricle. Simultaneously in systemic circulation , the left ventricle pumps blood to all parts of the body and returns back in the right auricle. Valves in heart check that the blood flows in one direction only. In this system, blood flows twice to complete a single cycle and is known as Double circulatory system. It is necessary for maintaining body temperature and collecting CO2 from cells and provising oxygen and energy to cells & tissues.