Set the
SaveFileDialog.OverwritePrompt
property to false, and it will not display any further information to the user.
The dialog itself does not affect files at all: even if the file exists it will not be affected in any way by the dialog, only by your code.
So after the ShowDialog call, check if the file exists:
SaveFileDialog sfd = new SaveFileDialog();
sfd.OverwritePrompt = false;
if (sfd.ShowDialog() == DialogResult.OK)
{
if (File.Exists(sfd.FileName))
{
...
}
}
You can then add the number yourself as needed.
(I would probably rename the existing file to "existingFileName.00001.originalextension" rather than adding a number to the newly saved file, but that's just me)