When you try to duplicate a quiz which has some questions with images or other files URLs like '<img src="http://moodle.devbox16.local/file.php/6/RspQ-2010%20Carbonyl%20Quiz%201/q6ethbr.gif" alt="Q6EthBr" align="bottom" border="0">' it will fail with  error_question_answers_missing_in_db

It happens because backup_xml_transformer::process_filephp_links on backup and restore_structure_parser_processor::process_cdata on restore treat $@FILEPHP@$ differently.

The first function replaces file path without a trailing slash, but the second function adds additional slash on restore.

Because of that restore_qtype_match_plugin::process_match can't match questiontext as data coming from XML has an extra slash and it fails with 'error_qtype_match_subquestion_missing_in_db' exception. 

 Steps to replicate:

1. Create a course.
2. Restore attached quiz activity into it
3. Try to duplicate an activity.
    

As a side effect every time you backup and restore the activity again it adds more extra slashes to the file paths.