Find duplicates in array field

From: Szaniszlo Szöke <szoke.szaniszlo@xxxxxxxxx>
To: mongodb-user <mongodb-user@xxxxxxxxxxxxxxxx>
Date: Mon, 25 Apr 2016 07:26:11 -0700 (PDT)
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Hi,

I get duplicates of fields and subfield using aggregation. Searching for 
duplicated sub field "k01.v" works fine:

db.table_0.aggregate({"$group" : { _id: "$k01.v", "count": { "$sum": 1 } } }
{"$match": {"count" : {"$gt": 1} } }
{"$project": {"k01.v" : "$_id", "_id" : 0} });

Unfortunately, this doesn't work when searching for sub field "a1.0" (field 
"a1" is an array):

db.table_0.aggregate({"$group" : { _id: "$a1.0", "count": { "$sum": 1 } } }
{"$match": {"count" : {"$gt": 1} } }
{"$project": {"a1.0" : "$_id", "_id" : 0} });

Any workaround ?

TIA

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Why ads?