In 4 stroke engines why inlet and outlet valves have two springs?

Two springs or concentric springs are provided with different size to help the closing of the valves. the reason for fitting two springs are that if one spring fails the valve will be held up by the other spring and prevent the valve from dropping down in the cylinder.

The spring may be vibrate axially and surge, When there is a frequency due to natural vibration of spring is in harmonic with cam shaft speed.

so for to reduce the incident of resonance, due to the  two springs are of different size have different vibrations characteristics and fitted on inside the other.