Capacitor duty contactors are special contactors which are used to switch on capacitors and capacitor banks. When a capacitor is switched, it initially appears as a short circuit. In a capacity duty contactor, this current is reduced by means of a series resistors and special circuits. 
Ordinary contactors will get damaged over repeated switching of the capacitors. Hence, only capacitor duty contactors should be used for switching on capacitors. The series resistors and contacts inside the unit can be replaced.
These special contactors limit the inrush current and thus reduce the stress inside the capacitor itself. This tends to increase the capacitor lifespan.