On an exterior wall in a heating climate, the poly behind the cement board can act as both the house's vapor barrier AND as the vapor barrier/drainage plane for the shower. To be the vapor barrier for the house, the poly should be continuous and any laps should be sealed. If the insulation in the wall behind the poly is faced (kraft facing, foil facing), render the facing ineffective by slashing it with a utility knife.

For interior walls where you just need a drainage plane, the seams in the poly can be simply lapped to shed water, but they don't need to be sealed to prevent vapor drive.

With a continuous and fully sealed plastic/visqueen membrane, you don't need Kerdi. If you did use Kerdi, then remove or slash the visqueen to render it useless as a barrier where the Kerdi and Visqueen overlap. You don't want overlapping non-permeable membranes in a wall assembly.