The answer anhic gave can be very inefficient if you have a large database and the attribute name is present only in some of the documents.
To improve efficiency you can add a $match to the aggregation.
db.collection.aggregate(
{"$match": {"name" :{ "$ne" : null } } },
{"$group" : {"_id": "$name", "count": { "$sum": 1 } } },
{"$match": {"count" : {"$gt": 1} } },
{"$project": {"name" : "$_id", "_id" : 0} }
)