Install error: DatabaseSchemaObjectExistsException

This is caused by the line

drupal_install_schema('uc_dotpay_pl');

in uc_dotpay_pl.install file.

In the documentation of hook_install()

http://api.drupal.org/api/drupal/modules!system!system.api.php/function/hook_install/7

there is

If the module implements hook_schema(), the database tables will be created before this hook is fired.

And the uc_dotpay_pl of course implements hook_schema().

By the way, it is also not necessary to call

drupal_uninstall_schema('uc_dotpay_pl');

in hook_uninstall(), because in Drupal 7 hook_uninstall() takes care about tables created by hook_schema().

http://api.drupal.org/api/drupal/modules!system!system.api.php/function/hook_uninstall/7