For example, you have an array of non-unique values that are student names, you know there are just 5 of them but a total of 15 elements exist in that array with some student names repeated in more than one elements.
To count the occurrences of all the unique values, in this case, 5 student names, in that array, simply use the PHP function array_count_values():
$counts = array_count_values($students);
The resulted array looks like:
Array ( ['George'] => 3 ['Clark'] => 2 ['John'] => 4 ['Annie'] => 1 ['Betsy'] => 5 )