E11000 duplicate key error index: MongoDb unusual error - mongodb

Question

I have a simple "users" collection inside which right now i have only 2 documents.
{
"_id": ObjectId("4ef8e1e41d41c87069000074"),
"email_id": {
"0": 109,
"1": 101,
"2": 64,
"3": 97,
{
"_id": ObjectId("4ef6d2641d41c83bdd000001"),
"email_id": {
"0": 109,
"1": 97,
"2": 105,
"3": 108,
now if i try to create a new index with {unique: true} on email_id field, mongodb complaints me with "E11000 duplicate key error index: db.users.$email_id dup key: { : 46 }". I get same error even after specifying {dropDups: true}, however i don't think this is the case here, as both document have different email id's stored.
I am not sure what's going on here, any pointers will be greatly appreciated.
Edit: Full view of documents:
{
"_id": ObjectId("4ef8e1e41d41c87069000074"),
"email_id": {
"0": 109,
"1": 101,
"2": 64,
"3": 97,
"4": 98,
"5": 104,
"6": 105,
"7": 110,
"8": 97,
"9": 118,
"10": 115,
"11": 105,
"12": 110,
"13": 103,
"14": 104,
"15": 46,
"16": 99,
"17": 111,
"18": 109
}
}
and
{
"_id": ObjectId("4ef6d2641d41c83bdd000001"),
"email_id": {
"0": 109,
"1": 97,
"2": 105,
"3": 108,
"4": 115,
"5": 102,
"6": 111,
"7": 114,
"8": 97,
"9": 98,
"10": 104,
"11": 105,
"12": 110,
"13": 97,
"14": 118,
"15": 64,
"16": 103,
"17": 109,
"18": 97,
"19": 105,
"20": 108,
"21": 46,
"22": 99,
"23": 111,
"24": 109
}
}
There are a couple of more fields like "display_name", "registered_since", etc which i have omitted from the display above (i don't think they have any role in the error thrown, if you still need them i can probably paste the entire documents here)
I am using erlang mongodb driver for communication with my mongo instance. All fields as can be seen are saved as binary bytes, thats why you see such weird email_id in document.
Note: Binary byte format is not forced by my code logic, i very much pass string email_id inside my bson documents, but i always end up seeing my data as binary bytes. (Probably because how erlang mongodb driver is written, i didn't really investigate on this, since my find(), find_one() and other queries works as expected even with fields saved as binary bytes)
Edit: > db.users.findOne()
{
"_id" : ObjectId("4ef6d2641d41c83bdd000001"),
"email_id" : [
109,
97,
105,
108,
115,
102,
111,
114,
97,
98,
104,
105,
110,
97,
118,
64,
103,
109,
97,
105,
108,
46,
99,
111,
109
],
"display_name" : [
65,
98,
104,
105,
110,
97,
118,
43,
83,
105,
110,
103,
104
],
"provider" : [
106,
97,
120,
108,
46,
105,
109
],
"provider_id" : [ ]
}

Answer 1

When MongoDB indexes an array field, it actually indexes the individual elements in the array. This is to efficiently support queries looking for a particular element of an array, like:
db.users.find({email_id: 46})
Since this email_id (46) exists in both documents, there are duplicate keys in your unique index.
I'm not sure why you would get this error if you have dropDups: true set... can you show a code sample with how you're invoking createIndex? You should also try dropDups: 1, as MongoDB erroneously treats 1 and true differently in this context (see https://jira.mongodb.org/browse/SERVER-4562).

Answer 2

For others having this problem, check your mongo version with db.version(). If you are running Mongo 3 and are trying to use dropDups to clear duplicates, it will fail and give you this error.