I would create a hash which keys would be the values the user has selected. If there are double entries, some of them will be overwritten. Just count how many keys your hash has, and if it's less than 5, that means entries are doubled (or tripled, or ...)

$entries{$INPUT{$_}} foreach (keys %INPUT);
if (%entries < 5) {
    $error = "duplicate vote";
    &error;
}
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