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");
}