IGBT's Ic (peak current): why always 2 times of Ic
On Semiconductor published a guide on how to interpret their IGBT datasheets. It says that: 


  The pulsed collector current describes the peak collector current pulse above the rated collector current specification that can flow while remaining below the maximum junction
  temperature. The maximum allowable pulsed current in turn depends on the pulse width, duty cycle and thermal conditions of the device. (P. 2)


So this is not some exact figure that is linked to the steady current by a factor of two; it is really a multi-dimensional limit. If a data sheet gives this figure for a specified pulse width, but doesn't give the duty cycle, there has to be some understood duty cycle. Obviously, the 1ms pulse cannot be repeated again and again, with just a microsecond of off time in between!

The assumption may be for a maximum duty cycle of 50%. If twice the current is turned on, but for half the time, and the thermal conditions are otherwise the same, junction temperature should be about the same, provided that the pulses are sufficiently narrow.