So when you are in the report of a quiz, mode overview, and you select the result of only a group of student, if some of them are enrolled multiple times through different enrollment methods, then it will count twice in the chart.
Full Steps :
— create a quiz in a course and enrol yourself as the teacher/manager.  

— enrol a student in the course twice through any methods (in the way where you see two active on top of each other).

— create a group and add the student to it.

— Take the quiz as the student.

— check the results, select Grades, select the group of the student.

— What's expected : at the bottom of page; two charts, one for the total of the quiz participant, and one for the group selected, both displaying the correct count of participants, 1 for each chart.
What actually happens: the group chart display double participant if the student is enrolled twice, so 1 in global chart and 2 in group chart.

The SQL_join in mod/quiz/report/reportslib.php l.202 seems to be in cause of the double counting.