たぶんこれでOK

inject.rb

require 'pp'

data = [
  {:code => 001, :item => {:name => 'jan'}},
  {:code => 001, :item => {:name => 'jan'}},
  {:code => 001, :item => {:name => 'aaa'}},
  {:code => 002, :item => {:name => 'feb'}},
  {:code => 002, :item => {:name => 'aab'}},
  {:code => 003, :item => {:name => 'mar'}},
  {:code => 003, :item => {:name => 'abb'}},
  {:code => 003, :item => {:name => 'foo'}},
  {:code => 004, :item => {:name => 'apr'}},
  {:code => 004, :item => {:name => 'bar'}},
  {:code => 004, :item => {:name => 'bar'}},
  {:code => 005, :item => {:name => 'may'}},
  {:code => 005, :item => {:name => 'baz'}},
  {:code => 005, :item => {:name => 'bbb'}}
]

uniq_list = []
data.uniq.inject(Hash.new){|prev, item| uniq_list << item if prev[:code] != item[:code]; item}
pp uniq_list

#[{:code=>1, :item=>{:name=>"jan"}},
# {:code=>2, :item=>{:name=>"feb"}},
# {:code=>3, :item=>{:name=>"mar"}},
# {:code=>4, :item=>{:name=>"apr"}},
# {:code=>5, :item=>{:name=>"may"}}]