[ruby-core:35403] Why are hash keys sometimes duped?

Why are some objects duped when they are used as hash keys and other
objects are not?

    irb(main):001:0> list = %w{ hello }
    => ["hello"]
    irb(main):002:0> str = "world"
    => "world"
    irb(main):003:0> map = { list => 1, str => 2 }
    => {["hello"]=>1, "world"=>2}
    irb(main):005:0> list << "world"
    => ["hello", "world"]
    irb(main):006:0> map
    => {["hello", "world"]=>1, "world"=>2}
    irb(main):007:0> str.replace("hi!")
    => "hi!"
    irb(main):008:0> str
    => "hi!"
    irb(main):009:0> map
    => {["hello", "world"]=>1, "world"=>2}
    irb(main):010:0> RUBY_VERSION
    => "1.9.3"
    irb(main):011:0>

-- 
Aaron Patterson
http://tenderlovemaking.com/