What is the coil voltage? Is it a DC 24V or AC voltage? If it is DC 24V coil and driven by the solid state output from the PLC then no need to add diode as there is built-in flyback diode. But if you use relay output of the PLC then you should add a diode to absorb the inductive kick.
However, if your coil is 120VAC and you are using the PLC's relay output (that's the only choice) then you can use a varistor to absorb the inductive spike. But if the current to drive the coil is low then use of varistor is not necessary.
