This is certainly well known, but not to me.

>> a  = [{"aa"=>"bb"},{"aa"=>"bb"}]
=> [{"aa"=>"bb"}, {"aa"=>"bb"}]

>> a.uniq
=> [{"aa"=>"bb"}, {"aa"=>"bb"}]

Why? and, what should I use instead of .uniq
to remove the duplicate?

Thank you
Piero