Black Swan employs many mirror scenes to illustrate how Nina struggles with her dual identity: the white swan and the black swan. In mirror you are supposed to see yourselves, but often Nina doesn’t see herself when she looks at the mirror. She is seeing her black swan counterpart. It is a common technique to illustrate bipolarity, I guess. The figure of Lily also serves similar function in the film. Lily, after all, is her doppelgänger for the role of the Swan Queen and she is also the projection of Nina’s alter ego. But Black Swan also adds another symbol to convey this idea: masturbation.
Nina is obsessed with masturbation and it functions similarly with the mirror. Masturbation, in this case, replaces ‘proper’ sex. Sex is supposed to be acted by two persons. Masturbation, on the other hand, collapses these multiplicity and replaces it by single agency. In masturbation scenes, we see two-yet-one Ninas, the white swan and the black swan, in action. Similarly, yet differently. In mirror scenes, Nina is one and becomes two. In masturbation scenes, Nina is two and becomes one.
(Lily, indeed, appeared in one of Nina’s masturbatory episodes where she fantasized making love with Lily although actually she was sexually arousing herself.)