  In this paper, we prove that the class of bi-f-harmonic maps and that of
f-biharmonic maps from a conformal manifold of dimension not equal to 2 are the
same (Theorem 1.1). We also give several results on nonexistence of proper
bi-f-harmonic maps and f-biharmonic maps from complete Riemannian manifolds
into nonpositively curved Riemannian manifolds. These include: any
bi-f-harmonic map from a compact manifold into a non-positively curved manifold
is f-harmonic (Theorem 1.6), and any f-biharmonic (respectively, bi-f-harmonic)
map with bounded f and bounded f-bienrgy (respectively, bi-f-energy) from a
complete Riemannian manifold into a manifold of strictly negative curvature has
rank < 2 everywhere (Theorems 2.2 and 2.3).
