When using the regenerate option to create new dataitems, there is a possiblitéy to generate dataitems that have similar itemnumber.
This do not invalidate the questions but these doublets should be eliminate if present and not generated.
This was unnoticed before the new messages on Moodle HEAD on database processes like this one

Did you remember to make the first column something unique in your call to get_records? Duplicate value 'a' found in column 'name'.

• line 567 of lib\dml\mysqli_native_moodle_database.php: call to debugging()
• line 1475 of question\type\calculated\questiontype.php: call to mysqli_native_moodle_database->get_records_sql()
• line 343 of question\type\calculated\questiontype.php: call to question_calculated_qtype->pick_question_dataset()
• line 1110 of lib\questionlib.php: call to question_calculated_qtype->create_session_and_responses()
• line 1151 of lib\questionlib.php: call to question_load_states()
• line 137 of question\preview.php: call to get_question_states()

A solution has been found but further tests need to be done before CVS.