Before assigning region in ServerShutdownHandler#process, it will check whether region is in RIT,
however, this checking doesn't work as the excepted in the following case:
1.move region A from server B to server C
2.kill server B
3.start server B immediately
Let's see what happen in the code for the above case
In fact, when running ServerShutdownHandler#process()#this.services.getAssignmentManager().processServerShutdown(this.serverName), region A is in RIT (step1.3 not completed), but the return List<RegionState> regionsInTransition doesn't contain it, because region A has removed from AssignmentManager.regions by AssignmentManager#setOffline in step 1.2
Therefore, region A will be assigned twice.
Actually, one server killed and started twice will also easily cause region assigned twice.
Exclude the above reason, another probability :
when execute ServerShutdownHandler#process()#MetaReader.getServerUserRegions ,region is included which is in RIT now.
But after completing MetaReader.getServerUserRegions, the region has been opened in other server and is not in RIT now.
In our testing environment where balancing,moving and killing are executed periodly, assigning region twice often happens, and it is hateful because it will affect other test cases.