Hi!

I have a quick question: Can a PC install Superior Weapon/Armor Customization more than once? Or if a weapon already is considered to be Superior quality, is it possible to grant Superior Customization to double the benefits?

I don't think is possible, but I don't see it in the rules. Thanks for your answers!

There's nothing to stack. It grants the Superior Quality. Not a "Rank" of Superior Quality. Same thing with trying to add multiple bipod mounts to a weapon.