[SOLVED] Duplicate data is sometimes inserted with unique index

Duplicate data is sometimes inserted with unique index

I do have an unique index with a partialFilterExpression on a collection but duplicate data is sometimes inserted.

Index creation

getCollection().createIndex(new BasicDBObject(userId, 1)
        , new BasicDBObject("name", "uidx-something-user")
                .append("partialFilterExpression", new BasicDBObject(Properties.someting, new BasicDBObject("$eq", true)))
                .append("unique", true));

The index from the getIndicies command

{
        "v" : 1,
        "unique" : true,
        "key" : {
                "userId" : 1
        },
        "name" : "uidx-something-user",
        "ns" : "somewhere.something",
        "partialFilterExpression" : {
                "something" : {
                        "$eq" : true
                }
        }
}

The duplicated Docuemnts

{
        "_id" : "08a8506c-bcbc-4ed6-9972-67fd7c37b4bc",
        "userId" : "1068",
        "express" : false,
        "something" : true,
        "items" : [ ],
        "recipient" : {
                "_id" : "efbd8618-c480-4194-964e-f5a821edf695"
        }
}
{
        "_id" : "b6695c6a-f29d-4531-96ac-795f14c72547",
        "userId" : "1068",
        "express" : false,
        "something" : true,
        "items" : [ ],
        "recipient" : {
                "_id" : "4f93fe38-edb2-4cb7-a1b3-c2c51ac8ded1"
}

MongoDb version: 3.2.7, seems also to happen with 3.2.12

A Sidenote: When dumping the collection and restoring it, a duplicate key error is thrown

Why is it sometimes possible to insert duplicate data and how to avoid that?

UPDATE

I created an MongoDb issue https://jira.mongodb.org/browse/SERVER-28153

Solution

Was fixed in 3.2.13, 3.4.4, 3.5.6 You cen read more in the mongodb jira