Weird vision issue...

I may have had faint symptoms of this for a while, but today I notice they're quite acute. The symptoms in question are fairly simple: it seems that the whole world has a second, shadowy "ghost", looking like a Photoshop "drop shadow" effect, vertically beneath it. With text the effect is quite striking; at normal distances, it appears literally like a drop shadow, nearly the height of the text itself and exactly duplicating the text, but much fainter. If I squint through my eyelashes the drop shadows go away but I'm not sure if that's actually making them go away, or changing the apparent contrast so I can't see them any more.

It appears to be the same in both eyes -- no apparently stronger or weaker effect in either eye -- and exists whether I wear my glasses or not.

I'm hoping this is going to go away -- maybe it's a matter of slightly irritated eyes or something. If it continues or gets worse it will become a significant impediment, though.