MongoDB is found to accept duplicate elements within the same level inside a document.
The issue is faced when inserting record into MongoDB from R 3.3.2 using rmongodb library.

The severity of this issue is that Mongodb didn't complain about it at all.
When trying to view the document inside Mongochef Studio 3T, you can't actually see the duplicate, instead you only see one element.

However, when trying to query it using C# driver and looping through the code generates an error complaining about a duplicate element.

When trying to view the document using Mongo Shell you can't see the duplicate either and you just see one element.

So basically you have duplicate inside your documents but you can't see it and you are not aware of it until you interrogate it using C# driver.

I hope this explains it, and I'm happy to answer any questions.