MongoDB: How to check for duplicates in a collection

This techniques to find out dupes in a MongoDB collection uses Map Reduce. The steps involved are simple:

Create a script called, say checkdupes.js add the following code to it.  The script runs on a collection called myCollection and examines the values of the field called myField, for each distinct value of myField, it inserts a document in a new collection called myDupesCollection.

m = function () {
    emit(this.myField, 1);
}

r = function (k, vals) {
    return Array.sum(vals);
}
res = db.myCollection.mapReduce(m, r, { out : "myDupesCollection" });

The script above can be run from the command line as follows

mongo myDB checkdupes.js

Now check for the dupes in the newly created collection by running the following command

db.myDupesCollection.find({value: {$gt: 1}});