When given duplicate pipeline names to a $facet stage, like

{ $facet: { a: [{$match: {a: 5}}], a: [{$match: {a: 0}}] } }

the current behavior is that $facet will just take the last pipeline with that name.

In SERVER-99210, we decided to reject $rankFusion queries that have duplicate pipeline names, so we filed this ticket to investigate if we should also have $facet reject those cases, or at least write tests to make sure the behavior is defined and deterministic.