Co-channel interference.

What is it?
This is where two or more stations are being received on the same frequency, at the same time. On a television, this may show up as one picture on top of another, or dark horizontal lines on the picture.

What can suffer from interference from these signals?
Any receiver can suffer, but it is quite often seen with televisions.

Who's fault is it?
This is absolutely NOTHING to do with CBs or other interference, it is usually just due to conditions at the time.

What causes it?
It is common for a TV or radio frequency to be allocated to more than one station, but usually distance between these stations means that only one will be received, depending upon which one is in your area.
Sometimes, weather conditions mean that signals may travel further than normal, and a distant station may interfere with your local station.

What should I do about it?
There is really nothing you can do except wait for the conditions to change.