For some applications,

  dict[key] << val
  dict[key].uniq!

will be good enough. But this will still slow down if the number of 
values in the list gets large, in which case a hash of hashes would be 
better:

  dict[key] ||= {}
  dict[key][val] = true

To get the values, you'd then use dict[key].keys (which in ruby 1.8 
would be in an arbitrary order, and in 1.9 would be in order of first 
insertion)

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