[{"_id":"anyID0","contentID":"content1","value":"any value","at":10},{"_id":"anyID1","contentID":"content1","value":"any value","at":9},{"_id":"anyID2","contentID":"content2","value":"any value","at":8},{"_id":"anyID3","contentID":"content3","value":"any value","at":7},]

db.collection.aggregate([{$group:{_id:"$contentID",first:{$first:"$$ROOT"}}},{$sort:{"first._id":1}},{$limit:5},{$replaceRoot:{newRoot:"$first"}},])