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$ cat test_case.rb

require 'mongo'

db = Mongo::Connection.new.db('sample-db')
coll = db.collection('test')
coll.remove

coll.insert({:_id => "test_add_to_set", :arr => []})
coll.update({:_id => "test_add_to_set"}, {:$addToSet => {:arr => {:$each => [1,1,2,3]}}})

puts coll.find({:_id => "test_add_to_set"}).next_document.inspect

$ ruby test_case.rb

{"_id"=>"test_add_to_set", "arr"=>[1, 1, 2, 3]}

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for some reason the value(1) in the array is repeated

is this a bug? or am I doing something wrong?