A simple function to check through a folder in the device if a file name already exists. If it does, append "(1)" to it to make it unique.

make_file_name.dart

// This function helps to add a string at the back of each file name IF that
// file already exists in the user's file system, so that each file name will
// be unique.
Future<String> makeFileName(String path, String fileName) async {
  bool fileExists = await File('$path/$fileName').exists();

  if (fileExists) {
    int counter = 1;
    List newFileExt = fileName.split('.');
    String ext = newFileExt[1]; // refers to the file extension.

    String fName = newFileExt[0];

    String newFileName = '$fName($counter).$ext';

    bool newFileExists = await File('$path/$newFileName').exists();

    while (newFileExists) {
      List newFileExt = fileName.split('.');

      String ext = newFileExt[1];

      String fName = newFileExt[0];

      /// The regex here is used to get the last occurrence of something like this: (1)
      /// (opening and closing bracket with digit(s) inside). The reason for this is that
      /// it may happen that a file name itself contains (something like this) "(1)"
      RegExp regExp = RegExp(r'\([0-9]+\)$');
      String? stringMatch = regExp.stringMatch(fName);

      if (stringMatch != null) {
        fName = fName.replaceAll(regExp, "(${counter + 1})");
      } else {
        fName = "$fName($counter)";
      }

      newFileName = '$fName.$ext';

      newFileExists = await File('$path/$newFileName').exists();
      counter += 1;
    }

    return newFileName;
  }

  return fileName;
}