  The two-pion production in pp-collisions has been investigated at CELSIUS in
exclusive measurements from threshold up to $T_p$ = 1.36 GeV. Total and
differential cross sections have been obtained for the channels $pn\pi^+\pi^0$,
$pp\pi^+\pi^-$, $pp\pi^0\pi^0$ and also $nn\pi^+\pi^+$. For intermediate
incident energies $T_p >$ 1 GeV, i.e. in the region which is beyond the Roper
excitation but at the onset of $\Delta\Delta$ excitation, the total
$pp\pi^0\pi^0$ cross section falls behind theoretical predictions by as much as
an order of magnitude near 1.2 GeV, whereas the $nn\pi^+\pi^+$ cross section is
a factor of five larger than predicted. An isospin decompostion of the total
cross sections exhibits a s-channel-like energy dependence in the region of the
Roper excitation as well as a significant contribution of an isospin 3/2
resonance other than the $\Delta(1232)$. As possible candidates the
$\Delta(1600)$ and the $\Delta(1700)$ are discussed.
