  A conjecture, related to the Nagata conjecture and the
Segre-Harbourne-Gimigliano-Hirschowitz conjecture, states that every integral
curve with negative self-intersection on the blow-up of $\P^2$ at a set of
points in very general position is a (-1)-curve. In this paper we verify this
conjecture for all curves whose image on $\P^2$ is a curve with a singularity
of multiplicity two at one of the centers of the blow-up.
