This mod will ensure that both halves, or all four quarters, of your world map are identical mirror images of each other, for multiplayer/challenge balance.

There are mod settings available to control mirroring on the X and Y axes, and to move the mirror location away from the origin (4 chunks by default).

It will only apply to newly generated chunks, and will produce discontinuities in existing maps. Decoratives do not mirror, and there are small bugs with cliffs.