A friend of mine asked me to get him snap caps for his double shotguns I never really thought of it but seems to make sense to release spring pressure

Is this the normal for doubles

Always have.

Using a spent shell works for me.

It's a good question. I never do, just didn't want anything in the chamber for grit or moisture to hold tight against while sitting there for any length of time. I'm probably over thinking it.

Actually mogc I had same thought while no spring tension there would be a snap cap to catch moisture or grit

Agree with tbear. I don't use them except for dry-fire practice. Not just grit to worry about, there's the galvanic corrosion potential (yeah, that's a pun).

Mine are all put away with hammers back. Flat or coil. Gun springs are always under some tension, and they wear through use cycles, not because of static compression.

On all but a very few guns, Lefever and L.C. Smith come to mind, it is simple to relieve the spring tension without using snap caps or spent shells.

Take forearm and barrels off the frame. Return the top lever to center via push button on the breech face or by lowering the bottom plate cocking arm (Brownings and similar). Then hold a plastic or similar material screw driver handle against the firing pin hole with pressure and snap the trigger. Repeat on second barrel. Then reassemble gun and put away.