When running a Jaspersoft report from from 'Reports and Jobs' page and selecting the format of XSLX, the report header repeats about every 16 - 18 rows.
This is currently working as designed. The reports have been hard coded to use the 'XLSX with Pagination' format when run from 'Reports and Jobs, ' which then adds a copy of the column headers on every page.
In order to export the report without the repeated headers, run the report from Advanced Reporting with the XSLX format instead.