  We consider an analytic family of Riemannian metrics on a compact smooth
manifold $M$. We assume the Dirichlet boundary condition for the
$\eta$-Laplacian and obtain Hadamard type variation formulas for analytic
curves of eigenfunctions and eigenvalues. As an application, we show that for a
subset of all $C^r$ Riemannian metrics $\mathcal{M}^r$ on $M$, all eigenvalues
of the $\eta$-Laplacian are generically simple, for $2\leq r< \infty$. This
implies the existence of a residual set of metrics in $\mathcal{M}^r$, which
makes the spectrum of the $\eta$-Laplacian simple. Likewise, we show that there
exists a residual set of drifting functions $\eta$ in the space $\mathcal{F}^r$
of all $C^r$ functions on $M$, which makes again the spectrum of the
$\eta$-Laplacian simple, for $2\leq r< \infty$. Besides, we give a precise
information about the complementary of these residual sets, as well as about
the structure of the set of deformations of a Riemannian metric (respectively
of the set of deformations of a drifting function) which preserves double
eigenvalues. Moreover, we consider a family of perturbations of a domain in a
Riemannian manifold and obtain Hadamard type formulas for the eigenvalues of
the $\eta$-Laplacian in this case. We also establish generic properties of
eigenvalues in this context.
