[{"_id":ObjectId("624dd0aca5fb565661da1165"),"name":"singleRecordInMongo","__v":0,"sections":[{"name":"firstElement","link":"https://www.test.com/","_id":ObjectId("624dd0aca5fb565661da1161")},{"name":"secondElement","link":"https://www.test.com/","_id":ObjectId("624dd0aca5fb565661da1162")},{"name":"thirdElement","link":"https://www.other.com","_id":ObjectId("624dd0aca5fb565661da1163")}]}]

db.collection.aggregate([{$unwind:"$sections"},{$group:{_id:"$sections.link",orig_id:{$first:"$sections._id"},count:{$sum:1}}},{$match:{"count":{$gt:1}}},{$group:{_id:0,sections:{$push:{link:"$_id",count:"$count"}}}}])