Under the top, between the light bulbs for the arrows, there is a piece of foam that blocks the light of one bulb from showing up in the arrow of the other. It's possible that this foam has deteriorated to the point that when one bulb goes on it shines through both arrows. Only way to know for sure would be to pop the top and check. At the same time you would see if both bulbs are actually coming on at the same time. As I remember the circuitry, however, I think that this is not possible.
Hope this helps...
Russ