  Photoproduction of $\pi^{0}\pi^{0}$ and $\pi^{0}\pi^{\pm}$ pairs from nuclei
has been measured over a wide mass range ($^2$H, $^{7}$Li, $^{12}$C, $^{40}$Ca,
and $^{\rm nat}$Pb) for photon energies from threshold to 600 MeV. The
experiments were performed at the MAMI accelerator in Mainz, using the Glasgow
photon tagging spectrometer and a 4$\pi$ electromagnetic calorimeter consisting
of the Crystal Ball and TAPS detectors. A shift of the pion-pion invariant mass
spectra for heavy nuclei to small invariant masses has been observed for
$\pi^0$ pairs but also for the mixed-charge pairs. The precise results allow
for the first time a model-independent analysis of the influence of pion
final-state interactions. The corresponding effects are found to be large and
must be carefully considered in the search for possible in-medium modifications
of the $\sigma$-meson. Results from a transport model calculation reproduce the
shape of the invariant-mass distributions for the mixed-charge pairs better
than for the neutral pairs, but also for the latter differences between model
results and experiment are not large, leaving not much room for
$\sigma$-in-medium modification.
