Hashes can't have duplicate keys. If you assign to a pre-existing hash key, the old value is simply overwritten. Consider the following simplistic example:

my %foo = (
           animal => 'monkey',
           fish   => 'tuna',
           insect => 'ant',
           insect => 'spider'
           );

while (my ($key, $value) = each %foo) {
    print "KEY: $key\n  VALUE: $value\n";
}
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This prints out the expected:

KEY: insect
  VALUE: spider
KEY: animal
  VALUE: monkey
KEY: fish
  VALUE: tuna
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Note that there is no ant in there. So I'm missing something obvious, or you'll be stomping on a value.