  We show that coarse graining arguments invented for the analysis of
multi-spin systems on a randomly triangulated surface apply also to the O(n)
model on a random lattice. These arguments imply that if the model has a
critical point with diverging string susceptibility, then either \g=+1/2 or
there exists a dual critical point with negative string susceptibility
exponent, \g', related to \g by \g=\g'/(\g'-1). Exploiting the exact solution
of the O(n) model on a random lattice we show that both situations are realized
for n>2 and that the possible dual pairs of string susceptibility exponents are
given by (\g',\g)=(-1/m,1/(m+1)), m=2,3,.... We also show that at the critical
points with positive string susceptibility exponent the average number of loops
on the surface diverges while the average length of a single loop stays finite.
