spilling question

Hi
  I am running into this spilling assertion when trying to insert a
store after the interval using
insertStoreAfterInstOnInterval.

Assertion failed: it == mi2iMap_.end() && "Already in map!", file C:\oxili-llvm\
llvm\include\llvm/CodeGen/LiveIntervalAnalysis.h, line 253

Isnt the instruction supposed to be in the map since it being spilled
? Any hints to what might be the reason for the assertion ?

thanks
shrey