CSV export producing different result in preview vs report server

In the specific instance that I'm trying to fix, I'm getting duplicate rows of data.

In the actual report (not the csv), there is a table and chart each displaying the same data. When I originally designed the report, I set the DataElementOutput property to 'NoOutput' on the chart, so that the CSV export would not display the data twice. That worked when exporting from the report previewer.

However, although the CSV exported from the Report Server is showing duplicate rows, it still appears to be coming from the table and not the chart. When DataElementOutput is set to 'auto' on the chart, there are subtle differences in the CSV output from the chart and table (for example, the CSV column names are the names that I assigned to the objects in the report) -- from these clues I can see that the DataElementOutput property is still working as expected. Nevertheless, duplicate rows appear.