rename file if already exists in database while uploading

I like to use the the way windows allows for unique file or folders: New Folder --> New Folder (2) if the folder exists.

string fileName = /* your file path */
string newFileName = fileName;
int count = 2;
while (System.IO.File.Exists(newFileName))
{
    if(fileName.EndsWith("\\")) fileName = fileName.Remove(fileName.Length-1);

    string dir = fileName.Substring(0,fileName.LastIndexOf("\\"));
    string fName = fileName.Replace(dir, "");
    string name = fName.Split('.')[0];
    string ext = fName.Split('.')[1];
    newFileName = dir + string.Format("{0} ({1}).{2}", name, count.ToString(), ext);
    count++;
}
// Here you would use your unique newFileName

I hope I understood you correctly