The pie chart shows redundant entries in the case that a custom field has more than one custom field context. In the attached screenshot you see a pie chart with statistic type on a single select list custom field. The select list entries are the same in the different contexts. The difference is in default values of single JIRA projects.

By clicking on one of the circle segments with the same name you get the sum of all issues that meet the filter criteria. This is as expected. But it is also expected that you have only one segment of the circle per name.