[R] plot() - thicker points when pair exist more than 1 time?

[Knut Krueger]

Hi to all,
is it possible to show in anyway that point 1,1 is existing more than 
1one time?
f.e.:
f<-  data.frame("x"=c(1,3,5,6,1),"y"=c(1,2,3,4,1))
plot(f)

Regards Knut