(*Mathematica 9*)

GraphPlot[{1->2,{2->3,"line1"},{2->3,"line2"},3->4},MultiedgeStyle->1.0,ImageSize->600](*works correctly in Mathematica 8*)

GraphPlot[{1->2,{2->3,"line1"},{3->2,"line2"},3->4},MultiedgeStyle->1.0,ImageSize->600]