From justin.seyster@mongodb.com 's code review of SERVER-80563 PR 15298, in reference to sbe_stage_builder.cpp buildUnwind():

"$unwind stages that request an index will need to materialize a BSON document twice: once to add the index and once to add the unwound field. Would it be practical to use one generateProject call that adds both fields in the same path?"

This ticket is to address that issue.