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When you tried to add the West Zone Relief Fan to the AirLoopHVAC:OutdoorAirSystem:EquipmentList object, you didn't change the syntax correctly. Here is your object in the IDF you provided:

AirLoopHVAC:OutdoorAirSystem:EquipmentList,
  West Data Center OA Equipment,                  !- Name
  OutdoorAir:Mixer,                               !- Component 1 Object Type
  West Data Center OAM Box;                       !- Component 1 Name
  Fan:VariableVolume,                             !- Component 2 Object Type
  West Zone Relief Fan,                           !- Component 2 Name

The last input field of all EnergyPlus objects should end with a semi-colon (;). You have a semi-colon after only the OutdoorAir:Mixer object's name West Data Center OAM Box, so EnergyPlus thinks that is the end of the OA system equipment list and the next two lines are defining a new Fan:VariableVolume object with the name West Zone Relief Fan. You actually define this fan object on line 1455 in this IDF, which is why EnergyPlus is giving you the severe error of duplicate object name (same VAV fan object type and same name defined in the IDF).

To fix this severe error, you should update this object as follows:

AirLoopHVAC:OutdoorAirSystem:EquipmentList,
  West Data Center OA Equipment,                  !- Name
  OutdoorAir:Mixer,                               !- Component 1 Object Type
  West Data Center OAM Box,                       !- Component 1 Name
  Fan:VariableVolume,                             !- Component 2 Object Type
  West Zone Relief Fan;                           !- Component 2 Name

From the Input Output Reference for the OA system equipment list object, the order that the components are listed is the order that EnergyPlus will simulate their airflow. If it doesn't seem that the relief fan is being controlled correctly, you may want to try switching the order of components in the OA system equipment list (relief fan is component #1, OA mixer is component #2).