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 $pnpi^+pi^0$, $pppi^+pi^-$, $pppi^0pi^0$ and also $nnpi^+pi^+$. For intermediate incident energies $T_p >$ 1 GeV, i.e. in the region which is beyond the Roper excitation but at the onset of $DeltaDelta$ excitation, the total $pppi^0pi^0$ cross section falls behind theoretical predictions by as much as an order of magnitude near 1.2 GeV, whereas the $nnpi^+pi^+$ 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.
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