Is there a NoSQL database that can effectively detect duplicate items?

I'm looking to implement a system that searches for duplicate entries before saving new entries, mostly by IP address. Since NoSQL databases have eventual consistency, this doesn't seem like a natural use case. Is there开发者_运维知识库 a way to make it work?

CouchDB enforces uniqueness within _id field of the document. Here's and excerpt from http://guide.couchdb.org

Within a CouchDB database, each document must have a unique _id field. If you require unique values in a database, just assign them to a document’s _id field and CouchDB will enforce uniqueness for you.

There’s one caveat, though: in the distributed case, when you are running more than one CouchDB node that accepts write requests, uniqueness can be guaranteed only per node or outside of CouchDB. CouchDB will allow two identical IDs to be written to two different nodes. On replication, CouchDB will detect a conflict and flag the document accordingly.

Every relational database and MongoDB supports unique indexes on data tables/collections preventing the insertion of duplicate data...why isn't that good enough?

Creating a unique index in MongoDB is straight forward. Trying to insert duplicate entries will raise an error (if you use safe mode enabled or checking the result of the insertion operation).