  Let $(M,g)$ be a closed locally conformally flat Riemannian manifold of
dimension $n \ge 7$ and of positive Yamabe type. If $\xi_0$ denotes a
non-degenerate critical point of the mass function we prove the existence, for
any $ k \ge 1$ and $\varepsilon >0$, of a positive blowing-up solution
$u_{\varepsilon}$ of $$\triangle_g u_{\varepsilon} +\big( c_n S_g +\varepsilon
h\big) u_{\varepsilon} = u_{\varepsilon}^{2^*-1},$$ that blows up like the
superposition of $k$ positive bubbles concentrating at different speeds at
$\xi_0$. The method of proof combines a finite-dimensional reduction with the
sharp pointwise analysis of solutions of a linear problem. As another
application of this method of proof we construct sign-changing blowing-up
solutions $u_{\varepsilon}$ for the Br\'ezis-Nirenberg problem $$
\triangle_{\xi} u_{\varepsilon} - \varepsilon u_{\varepsilon} =
|u_{\varepsilon}|^{\frac{4}{n-2}} u_{\varepsilon} \textrm{ in } \Omega, \quad
u_{\varepsilon} = 0 \textrm{ on } \partial \Omega $$ on a smooth bounded open
set $\Omega \subset \mathbb{R}^n$, $n \ge 7$, that look like the superposition
of $k$ positive bubbles of alternating sign.
