> db.foo.drop()
true
> for (var i=0; i<100; i+) for (var j=0; j<100; j+) db.foo.insert({locs:{loc:[[i+Math.random(), j+Math.random()],[i+Math.random(), j+Math.random()],[i+Math.random(), j+Math.random()],[i+Math.random(), j+Math.random()],[i+Math.random(), j+Math.random()],]}})
> db.foo.ensureIndex(

{'locs.loc':'2d'}

)
> db.foo.find({'locs.loc': {$near: [50,50]}}, {_id:1}).limit(10)

{ "_id" : ObjectId("4e5e9db248cf912e7af1a478") } { "_id" : ObjectId("4e5e9db248cf912e7af1a414") } { "_id" : ObjectId("4e5e9db248cf912e7af1a414") } { "_id" : ObjectId("4e5e9db248cf912e7af1a478") } { "_id" : ObjectId("4e5e9db248cf912e7af1a414") } { "_id" : ObjectId("4e5e9db248cf912e7af1a415") } { "_id" : ObjectId("4e5e9db248cf912e7af1a479") } { "_id" : ObjectId("4e5e9db248cf912e7af1a478") } { "_id" : ObjectId("4e5e9db248cf912e7af1a479") } { "_id" : ObjectId("4e5e9db248cf912e7af1a415") }

Note that we do dedupe on normal indexes:
> db.bar.insert(

{a:[1,2,3,4]}

)
> db.bar.insert(

{a:[1,2,3,4]}

)
> db.bar.find({a:{$in:[1,2,3,4]}})

{ "_id" : ObjectId("4e5e9ee648cf912e7af1b7d0"), "a" : [ 1, 2, 3, 4 ] } { "_id" : ObjectId("4e5e9ee848cf912e7af1b7d1"), "a" : [ 1, 2, 3, 4 ] }