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Online Sandbox for SQL and PHP: Write, Run, Test, and Share SQL Queries and PHP Code
<?php
$arr = [1,2,3,4,4,4,5,6,7,8,9,9,99];
// Считаем количество значений
$count_values = array_count_values($arr);
// Фильтруем те котовых больше 1
$result = array_diff ( $count_values, [ 1 ] );
$result = array_keys($result);
print_r($result);