Index: lams_common/src/java/org/lamsfoundation/lams/util/ExcelUtil.java
===================================================================
diff -u -rfe1ae54fb42280aab925e53a727aee69a9387748 -rc29e529815c22e46dbf7711ddb9fcd62f9ec880d
--- lams_common/src/java/org/lamsfoundation/lams/util/ExcelUtil.java	(.../ExcelUtil.java)	(revision fe1ae54fb42280aab925e53a727aee69a9387748)
+++ lams_common/src/java/org/lamsfoundation/lams/util/ExcelUtil.java	(.../ExcelUtil.java)	(revision c29e529815c22e46dbf7711ddb9fcd62f9ec880d)
@@ -37,6 +37,7 @@
 import org.apache.poi.ss.usermodel.Row;
 import org.apache.poi.ss.usermodel.Sheet;
 import org.apache.poi.ss.usermodel.Workbook;
+import org.apache.poi.ss.util.WorkbookUtil;
 import org.apache.poi.xssf.streaming.SXSSFWorkbook;
 
 /**
@@ -142,6 +143,14 @@
 
     public static void createSheet(Workbook workbook, String sheetName, String sheetTitle, int sheetIndex,
 	    String dateHeader, ExcelCell[][] data) throws IOException {
+	
+	// Modify sheet name if required. It should contain only allowed letters and sheets are not allowed with
+	// the same names (case insensitive)
+	sheetName = WorkbookUtil.createSafeSheetName(sheetName);
+	while (workbook.getSheet(sheetName) != null) {
+	    sheetName += " ";
+	}
+	
 	Sheet sheet = workbook.createSheet(sheetName);
 
 	// Print title in bold, if needed