Ytukyg

First of all this is not a duplicate as i haven't found any info about this.
We can successfully remove duplicate values using the following from an array for example:

 $messages=   Array ( 

        [0] => Array ( [user] => 2224 [sending_time] => 1536513903 [read_time] => 1536513941 [content] => sad [recipient_status] => read ) 

        [1] => Array ( [user] => 3310 [sending_time] => 1536513903 [read_time] => 1536513941 [content] => sad [recipient_status] => read ) 

        [2] => Array ( [user] => user1 [sending_time] => 1536513874 [read_time] => 1536567672 [content] => def [recipient_status] => read ) 

        [3] => Array ( [user] => user1 [sending_time] => 1536513532 [read_time] => 1536513745 [content] => abc [recipient_status] => read ) 

    )

Using the following i can remove the duplicates(by key):

        $MSGS_array  = array();

        foreach ($messages as $message) {

            $MSGS_array[$message['user']] = $message;

        }

But the question is how can i get the removed user key value?

OR

How can i get all the duplicates having key user and their value(s) in an array from the array above?

Expected Output:

The output should only contain the removed/duplicates Like:(i just need the duplicates with they key user)

 $output=   Array (

        [0] => Array ( [user] => user1 ) 

        [1] => Array ( [user] => user1 ) 

 );

OR

 $output=   Array (

        [0] => Array ( [user] => user1 [sending_time] => 1536513874 [read_time] => 1536567672 [content] => def [recipient_status] => read ) 

        [1] => Array ( [user] => user1 [sending_time] => 1536513532 [read_time] => 1536513745 [content] => abc [recipient_status] => read ) 

 );

Here is one way to find the duplicate messages. First we find the non-unique users by checking the count of the values of user. Then we filter the messages in $messages by seeing if the user is in the non-unique users array:

$non_unique_users = array_filter(array_count_values(array_column($messages, 'user')), function ($v) { return $v != 1; });

$duplicate_messages = array_filter($messages, function ($v) use($non_unique_users) { return array_key_exists($v['user'], $non_unique_users); });

print_r($duplicate_messages);

Output:

Array ( 

    [2] => Array ( [user] => user1 [sending_time] => 1536513874 [read_time] => 1536567672 [content] => def [recipient_status] => read ) 

    [3] => Array ( [user] => user1 [sending_time] => 1536513532 [read_time] => 1536513745 [content] => abc [recipient_status] => read ) 

)

Demo on 3v4l.org

Update

To just return an array of the user keys, you can apply array_map to the $duplicate_messages array:

$duplicate_users = array_map(function ($v) { return array('user' => $v['user']); }, $duplicate_messages);

or you can derive it directly from the $messages and $non_unique_users arrays:

$duplicate_users = array_map(function ($v) use($non_unique_users) { if (array_key_exists($v['user'], $non_unique_users)) return array('user' => $v['user']); }, $duplicate_messages);

In both cases the output is

Array ( 

    [2] => Array ( [user] => user1 ) 

    [3] => Array ( [user] => user1 ) 

)

Demo on 3v4l.org