Why is my scene rendered repeatedly when I press F12? I hit F12, tiles all render out - just as the final image appears, the whole lot disappears only for it to start all over again.

I've not specified multiple frames or any animation, so can anyone explain why this happens and what it's for?
If your scene has more than one render layer, blender will render each one successively. Finishing one and starting over with the next one until the information of all layers is complete.