You can just keep track of the words that you've seen so far and replace as you go.
// words we've seen so far
$words_so_far = array();
// for each word, check if we've encountered it so far
// - if not, add it to our list
// - if yes, replace it
foreach($charset as $k => $word){
if(in_array($word, $words_so_far)){
$charset[$k] = $your_replacement_here;
}
else {
$words_so_far[] = $word;
}
}
For a somewhat-optimized solution (for cases where there are not that many duplicates), use array_count_values() (reference here) to count the number of times it shows up.
// counts the number of words
$word_count = array_count_values($charset);
// words we've seen so far
$words_so_far = array();
// for each word, check if we've encountered it so far
// - if not, add it to our list
// - if yes, replace it
foreach($charset as $k => $word){
if($word_count[$word] > 1 && in_array($word, $words_so_far)){
$charset[$k] = $your_replacement_here;
}
elseif($word_count[$word] > 1){
$words_so_far[] = $word;
}
}