[ MongoDB FAQ ] How to stop insertion of Duplicate documents in a mongodb collection

Source From Here 
Question 
Let us have a MongoDB collection which has three docs.. 
db.collection.find() 

{ _id:'...', user: 'A', title: 'Physics', Bank: 'Bank_A' }
{ _id:'...', user: 'A', title: 'Chemistry', Bank: 'Bank_B' }
{ _id:'...', user: 'B', title: 'Chemistry', Bank: 'Bank_A' }

We have a doc, 

doc = { user: 'B', title: 'Chemistry', Bank:'Bank_A' }

If we use 

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1. db.collection.insert(doc)   

here, this duplicate doc will get inserted in database. 

{ _id:'...', user: 'A', title: 'Physics', Bank: 'Bank_A' }
{ _id:'...', user: 'A', title: 'Chemistry', Bank: 'Bank_B' }
{ _id:'...', user: 'B', title: 'Chemistry', Bank: 'Bank_A' }
{ _id:'...', user: 'B', title: 'Chemistry', Bank: 'Bank_A' }

How this duplicate can be stopped. On which field should indexing be done or any other approach? 

How-To 
Don't use insert. 

Use update with upsert=true. Update will look for the document that matches your query, then it will modify the fields you want and then, you can tell it upsert:True if you want to insert if no document matches your query. 

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1. db.collection.update(  
2.    ,  
3.    ,  
4.   {  
5.     upsert: <boolean>,  
6.      multi: <boolean>,  
7.     writeConcern:   
8.    }  
9.   )  

So, for your example, you could use something like this: 

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1. db.collection.update(doc, doc, {upsert:true})  

Supplement 
* [ MongoDB 文件 ] Getting Started - Update Data with Java Driver