While we were implementing SBE support for the $in operator for the find() command, the question arose as to whether we want to rewrite the generatePathTraversal() function (in "src/mongo/db/query/sbe_stage_builder_filter.cpp") so that it only calls the “makePredicate” callback once when mode == kArrayAndItsElements .

It's unfortunate that generatePathTraversal() calls the “makePredicate” callback twice when mode == kArrayAndItsElements , because if the “makePredicate” callback adds stages to the tree (which we do for InMatchExpressions in some cases) then we end up have two copies of these stages in tree.