Change to event

Hi,
To this event

        public static void CreateSpreadsheetWorkbook(string filepath,string SheetName0)
        {
                SpreadsheetDocument spreadsheetDocument;
                if (System.IO.File.Exists(filepath) == false)
                {
                    // Create a spreadsheet document by supplying the filepath.
                    // By default, AutoSave = true, Editable = true, and Type = xlsx.
                    spreadsheetDocument = SpreadsheetDocument.
                        Create(filepath, SpreadsheetDocumentType.Workbook);
                }
                else
                {
                    spreadsheetDocument = SpreadsheetDocument.Open(filepath, false);
                }

                // Add a WorkbookPart to the document.
                WorkbookPart workbookpart = spreadsheetDocument.AddWorkbookPart();
                workbookpart.Workbook = new DocumentFormat.OpenXml.Spreadsheet.Workbook();

                // Add a WorksheetPart to the WorkbookPart.
                WorksheetPart worksheetPart = workbookpart.AddNewPart<WorksheetPart>();
                worksheetPart.Worksheet = new DocumentFormat.OpenXml.Spreadsheet.Worksheet(new SheetData());

                // Add Sheets to the Workbook.
                DocumentFormat.OpenXml.Spreadsheet.Sheets sheets = spreadsheetDocument.WorkbookPart.Workbook.
                    AppendChild<DocumentFormat.OpenXml.Spreadsheet.Sheets>(new DocumentFormat.OpenXml.Spreadsheet.Sheets());

                // Append a new worksheet and associate it with the workbook.
                Sheet sheet = new Sheet()
                {
                    Id = spreadsheetDocument.WorkbookPart.
                        GetIdOfPart(worksheetPart),
                    SheetId = 1,
                    Name = SheetName0
                };
                sheets.Append(sheet);

                workbookpart.Workbook.Save();

                // Close the document.
                spreadsheetDocument.Close();
        }

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I only want to append SheetName0 as a new worksheet to the Workbook, if SheetName0 does not exist within the workbook. And no change is applied to workbook, if SheetName0 is already existing inside the workbook. What to adjust?