Inserting duplicate values into a table using $wpdb

I have a table named wp_q33uds_campaign in my MySQL database, which is defined using a plugin as follows:

$sql = "CREATE TABLE `$tablename`(
            `user_id` INT(20) NOT NULL AUTO_INCREMENT,
            `tweet1` VARCHAR(200) NOT NULL,
            `date1` VARCHAR(200) NOT NULL,
            `tweet2` VARCHAR(200) NOT NULL,
            `date2` VARCHAR(200) NOT NULL,
            `tweet3` VARCHAR(200) NOT NULL,
            `date3` VARCHAR(200) NOT NULL,
            `tweet4` VARCHAR(200) NOT NULL,
            `date4` VARCHAR(200) NOT NULL,
            `tweet5` VARCHAR(200) NOT NULL,
            `date5` VARCHAR(200) NOT NULL,
            `flag` INT(10) NOT NULL,
            `created` DATETIME,
            );";

Now, I want the same user to be able to insert multiple entries into the table ie multiple entries of the same user_id should be allowed to be inserted into the table.

The code for inserting values into the table:

if($_POST['submitted'])
{
     $table_name = $wpdb->prefix."campaign";

        $new_data = array(
                'user_id' => 4344,
                'tweet1' => $_POST['tweet1'],
                'date1' => strtotime($_POST['date1']) - 19800,
                'tweet2' => $_POST['tweet2'],
                'date2' => strtotime($_POST['date2']) - 19800,
                'tweet3' => $_POST['tweet3'],
                'date3' => strtotime($_POST['date3']) - 19800,
                'tweet4' => $_POST['tweet4'],
                'date4' => strtotime($_POST['date4']) - 19800,
                'tweet5' => $_POST['tweet5'],
                'date5' => strtotime($_POST['date5']) - 19800,
                'flag' => 1,
                'created' => current_time('mysql'),
            );
        //print_r($new_data);

        $wpdb->insert($table_name, $new_data);
}

However, the wp_q33uds_campaign table doesn't accept entries with an user_id already present in the table. THe table does accept entries with user_id not present in the table.

How can I allow my table to accept entries with a duplicate user_id?