A snubber circuit will do. A capacitor and resistor in series for each contact. About 10...220 ohm and 10... 470nF. I use 47 ohm and 100nF. The capacitor must be high voltage.
I have seen relay contacts melting away over a period of a couple of days by arcs, so it can be a real danger.
If you have two outputs, you could make the SSR the first to close and the last to open. With a delay of 500ms or so.
But then again, a single heavy SSR will solve this problem.