[{"_id":0,title:"Movie A",year:2010,director:"D1",actors:["A1","A2"]},{"_id":1,title:"Movie B",year:2010,director:"D1",actors:["A1","A5","A6"]},{"_id":2,title:"Movie C",year:2010,director:"D2",actors:["A4"]},{"_id":3,title:"Movie D",year:2012,director:"D3",actors:["A7"]},{"_id":4,title:"Movie E",year:2015,director:"D3",actors:["A7"]}]

db.collection.aggregate([{$group:{_id:"$director",count:{$sum:1},rootDocs:{$push:"$$ROOT"}}},{$match:{count:{$gt:1}}},{$unwind:"$rootDocs"},{$replaceRoot:{newRoot:"$rootDocs"}}])