Suppose you're combining two factors when creating your facets (clarity and colour in this instance... subsetted just to make the example plot a little smaller):
ggplot(subset(diamonds, clarity=="SI1" | clarity == "VS2"),
aes(x=price,y=carat)) + geom_point() + facet_grid(~ clarity + color)
This gets you:
Is there any way to change the labelling so that the redundant SI1 and VS2 labels are reduced to one a piece?