Check duplicate in an array and return duplicate positions

I don't know Laravel Excel, but i assume you have the excel/csv date in some kind of array? If so, you can easily filter out the duplicates? There are several possibilities depending on your data structure. array_unique, loop while skipping the already added rows, etc. (with the last solution you would also get the affected rows).. why do you need the position of the duplicate row?

@StefanBauer I got the username and email arrays of users from excel file. I know i can use array_unique to check duplicate in each arrays but i want to show where the duplicate position to users so they can fix it.

for ($i = 0; $i < count($users) - 1; $i ++)
            {
                
                for ($j = $i + 1; $j < count($users); $j ++)
                {
                    if ($users[$i]['ten_tai_khoan'] == $users[$j]['ten_tai_khoan'])
                    {
                        $errors_username[] = $i ."-" .$j;

                    }
                    if ($users[$i]['email'] == $users[$j]['email'])
                    {
                        $errors_emails[] = $i ."-" .$j;
                    }
                }
                
            }

This works but i want to show all duplicate positions and reduce the loop to increase performance

Just use the first loop. Create an array like $processedUsernames = []; and fill the array in each loop with the username you processed. Before doing that, check if the username you are processing is in the $processedUsernames array. If so, you have the record (and therefore the row) which is a duplicate. You can create a new array, where you save informations about the duplicates. Like $duplicates[] = $users[$i]; or something..