Gradebook

When I click grade report, it went to the script like http://moodle_site/grade/report/grader/index.php?id=40
and then it got error msg to see found more than one record in fetch(), see the attached picture. I don't know what it was about and how to fix it.
It worked fine to view the report before.
Is there any one has the same experience and has solution?

Thanks
Ye