This can be caused by more than one answer being retrieved for the same workflow step.
Possible causes of this are:
Two different questions from the same form being mapped to one of the following filters:
Child Visits (Document).
Child Visit Seen Alone.
Child Visit Planned Date (Document).
Child Visit Actual Date (Document).
Duplicate Forms in a workstep. Please see the following article for further information: Duplicate Forms in a workstep
📌 Note: There was a similar error in a different populate step in earlier versions due to an incompatible Address Base epoch being in place, if you are on a Address Base version earlier than epoch 100, please update the Address Base version and check if the error is still present following a further populate job run.