You might be getting several messages like this when you try to uninstall and reinstall a migration module:

exception 'Drupal\Core\Config\PreExistingConfigException' with message 'Configuration objects (migrate_plus.migration.article) provided by migrate_MODULE_NAME already exist in active configuration' in /var/www/public/example.com/drupal/core/lib/Drupal/Core/Config/PreExistingConfigException.php:65

One way of solving it is through the console:

drush php;
Drupal::configFactory()->getEditable('migrate_plus.migration.migrate_article')->delete();
Drupal::configFactory()->getEditable('migrate_plus.migration.migrate_page')->delete();
Drupal::configFactory()->getEditable('migrate_plus.migration.migrate_another_content_type')->delete();
exit;

But adding an install file to your migration module, will take care of any lingering migrate configurations, and delete them on uninstall:

/**
 * @file
 * Contains MODULE_NAME.install.
 */

/**
 * Implements hook_uninstall().
 *
 * Removes stale migration configs during uninstall.
 */
function MODULE_NAME_uninstall() {
  $database = \Drupal::database();
  $query = $database->select('config', 'c')
    ->fields('c', array('name'))
    ->condition('name', $database->escapeLike('migrate_plus.') . '%', 'LIKE')
    ->execute();

  $config_names = $query->fetchAll();

  // Delete each config using configFactory.
  foreach ($config_names as $config_name) {
    \Drupal::configFactory()->getEditable($config_name->name)->delete();
  }
}

Original .install file found at Is there a way with drush or console to delete all active config with a certain prefix?