[{"fieldA":1,"fieldB":1},{"fieldA":2,"fieldB":3},{"fieldA":2,"fieldB":2},{"fieldA":4,"fieldB":5},{"fieldA":10},{"fieldB":null},{"fieldB":null}]

db.collection.aggregate([{$match:{$and:[{"fieldA":{$exists:true}},{"fieldB":{$exists:true}},{$expr:{$eq:["$fieldA","$fieldB"]}}]}}])