  Dual meson condensates as possible order parameters for deconfinement are
investigated in a Polyakov-loop enhanced linear sigma model of QCD at both zero
and finite isospin chemical potential $\mu_I$. We find that the rapid rise of
the dual sigma condensate (corresponding to the dressed Polyakov-loop) with $T$
is driven by the chiral transition, no matter whether the Polyakov-loop
dynamics is included or not. For $\mu_I>m_\pi/2$, the dual sigma condensate
shows abnormal thermal behavior which even decreases with $T$ below the melting
temperature $T_c^{I_3}$ of pion superfluidity; On the other hand, even the dual
pion condensate always increases with $T$, its maximum slope locates exactly at
$T_c^{I_3}$ rather than the deconfinement temperature $T_c^{P}$ determined by
the Polyakov-loop. All these are qualitatively consistent with the previous
results obtained in the Nambu-Jona-Lasinio type models. The dual vector meson
condensate for $\mu_I>m_\pi/2$ is also calculated. This quantity is more
sensitive to the chiral transition when taking into account the Dirac-sea
contribution. Our study further suggests that it should be cautious to use dual
observables to indicate the deconfinement transition, especially in QCD models.
