How to make user's email unique in mongoDB?

Use unique keyword, or simply make _id value to be email.

db.collection.createIndex( { email: 1 }, { unique: true } )

You can do it by creating a unique index for your email field.

http://docs.mongodb.org/manual/tutorial/create-a-unique-index/

Mongodb ensureIndex has been deprecated, use createIndex instead.

db.collection.createIndex( { email: 1 }, { unique: true } )