function onOpen() {
var activeSpreadsheet = SpreadsheetApp.getActiveSpreadsheet();
var yourNewSheet = activeSpreadsheet.getSheetByName("Name of your new sheet");
if (yourNewSheet != null) {
activeSpreadsheet.deleteSheet(yourNewSheet);
}
yourNewSheet = activeSpreadsheet.insertSheet();
yourNewSheet.setName("Name of your new sheet");
}