In the current releases and development version of Moodle, if an assignment in a given course and given category has the same name and is repeated 3 or more times, the student's grade is incorrectly computed (it inflates the student grade).
I have posted a bug report and fix here: http://tracker.moodle.org/browse/MDL-6876
As a workaround until the bug is fixed, do not use the same name for assignments in a given course.