MS-Excell can open/edit a POI/HSSF generated spreadsheet that contains sheets with duplicate names, but on closing or saving the spreadsheet it crashes everytime (version tested: Excel 2000, 9.0.3821 SR-1). Solution: avoid creating sheets with duplicate names by letting method workBook.createSheet(sheetName) throw an exception when it creates a sheet with a name that already exists.
Done some time ago: java.lang.IllegalArgumentException: The workbook already contains a sheet of this name at org.apache.poi.hssf.usermodel.HSSFWorkbook.createSheet(HSSFWorkbook.java:388) Jason