You probably don't need the duplicate carbon, but I guess it can't hurt.

I'm thinking of buying a second RO system for lake water at our cabin and there's one for sale used locally.

Is the diffence in most of these systems in the filters and cartridges? They look like they all are about the same size, so buying a used system and replacing the consumables with high grade sounds like maybe a good way to go?

Or are there different *standards* for these things as well?

10" canisters are standard. The RO membranes are also pretty standard, so everything is usually replacable, but for the used one you are looking at you should verify that they are 10" canisters, then you can buy replacements.

The only real differences between units are build quality, and some features like built in TDS, build in hydraulic switch, pressure gauge, and the total GPD the unit can pump out.