This is a very strange issue. First I would back up all of your photos and then format the memory card using the camera's menus. Then take more pictures and see if it's still a problem.
If that doesn't fix it, try turning off the stabilization features or any effects (a mirror type effect could produce this but seems unlikely on a higher-end camera like this).
If it's still a problem you will likely need to get it repaired by Sony, it sounds like a software issue.
In the meantime, it's easy to fix those photos using any basic image editor. You can try
GIMP (more advanced) or
Paint.net (more basic). Both are free. Just cut the bottom half and paste it on the top.
If you have Photoshop, you could even make an action to batch-fix all of your photos that have this problem.