Drupal require_once conflict

Sometimes you need to include a library that already exists and you think require_once() should solve it, but it doesn't. It will give you an Fatal error saying that you Cannot redeclare some function because its previously declared

Common libraries should not be installed in the module it should be installed in sites/all/default/libraries or something like that. But one other solution is to wrap the require_once() function with a function_exists().