Fixed a tricky bug in the scenario calculator with duplicate imts

I found this bug in the scenario_damage calculation for Portugal. In that calculation the IMTs are inferred from the exposure file and they are written in an inconsistent way: sometimes one finds the string SA(0.8), sometimes the SA(0.80). The engine consider them as distinct IMTs and save the same ground motion values twice. The solution is to collapse the IMTs to the really distinct ones. See https://bugs.launchpad.net/oq-engine/+bug/1320115.