Database
[{"_id":"1235","details":[{"productId":"PRODUCT1","myList":["0001","0001","0002","0002",]},{"productId":"PRODUCT2","myList":["0002","0002","1234","1234",]}]},{"_id":"12356","details":[{"productId":"PRODUCT1","myList":["0001","0001","0002","0002",]},{"productId":"PRODUCT2","myList":["0002","0002","1234","1234",]}]}]
Query
db.collection.aggregate([{$unwind:"$details"},{$group:{_id:"$_id",details:{$push:{productId:"$details.productId",myList:{$setUnion:"$details.myList"}}}}}])