[{"_id":ObjectId("5bcc0a44f2752576a8545d99"),"Teacher_id":"Pic002","Teacher_Name":"Ravi Kumar","Dept_Name":"IT","Salary":40000,"Status":"A"},{"_id":ObjectId("5bcc0a5af2752576a8545d9a"),"Teacher_id":"Pic003","Teacher_Name":"Akshay","Dept_Name":"Comp","Salary":25500,"Status":"N"},{"_id":ObjectId("5bcc0a85f2752576a8545d9b"),"Teacher_id":"Pic003","Teacher_Name":"Akshay","Dept_Name":"Comp","Salary":25500,"Status":"N"},{"_id":ObjectId("5bcc0a9af2752576a8545d9c"),"Teacher_id":"Pic004","Teacher_Name":"Sumit","Dept_Name":"Mech","Salary":35000,"Status":"N"}]

db.collection.aggregate([{$group:{_id:"$Dept_Name",docs:{$addToSet:"$$CURRENT"},count:{$sum:1}}},{$match:{"count":{"$eq":1}}},{$unwind:"$docs"},{$replaceRoot:{newRoot:"$docs"}}])