[{_id:1,field:{product:{_id:5}}},{_id:2,field:{product:{_id:6}}},{_id:3,field:{product:{_id:5}}},{_id:4,field:{product:{_id:1}}},]

db.collection.aggregate([{$group:{_id:"$field.product._id",firstRoot:{$first:"$$ROOT"}}},{$replaceRoot:{newRoot:"$firstRoot"}}])