Occasionally I understand something in #ggplot. So I had x-y data and a grouping category. When I added a smoother, two of them appeared -- for the two category levels; however I wanted only one. After considerable deliberation, I found the solution to be very easy. I had (simplified):

ggplot(data=d, aes(x=x, y=y, shape=group) +

geom_point() + stat_smooth()

Moving the group into a separate aesthetic helped:

ggplot(data=d, aes(x=x, y=y) +

geom_point(aes(shape=group) + stat_smooth()

To many of you it will be obvious, after all, it follows from the logic behind ggplot. Still, took me a while, now I'm happy.