Even for a small resin object like the grenade Adam Savage was talking about, if you're only going to make 1 for your "spare", it doesn't make financial sense. Just the silicone and resin you need will set you back that $100 a "legit" 2nd grenade would cost. And then you'd have resin and silicone left overs that you don't need. And it will quickly go bad and become useless.
So you'd have to make more than 1 spare. And what are you going to do with all the spares you made just to use up all that leftover resin? Surely you don't need 10 spares? Sell them? Boom, you've just become the bad kind of recaster. You could give them away to your friends who might be doing the same costume. It makes sense, right? But then the guy you've recast looses out on potential customers, and that's almost as bad as selling them yourself.
So there's a very, very narrow corridor of scenarios where you can justify recasting, and not either go immoral or put up with a great loss of money and/or time + effort instead of just supporting the artists themselves and keep living a cosy life yourself.