You can do two things.

1)  You could use something like File.Exists to see if the file already exists, if it does then append a number on the filename, for instance MyFile.txt (1), MyFile.txt (2), etc.
2)  You could generate a unique name regardless of what it was originally named using the Guid class.  You won't be able to tell what each file is, but you won't have conflicts.

 Hello,

 uptill now i able to save the files to the server with thier original names, but i want to save the files with different names to avoid duplication of file names. see the following for what i have acheived uptill now :
 strFileName = txtFileName.PostedFile.FileName
 strFileName = System.IO.Path.GetFileName(strFileName)
  'Save Uploaded file to server
  strFileNamePath = strFileFolder & strFileName

  If Not (txtFileName.PostedFile Is Nothing) Then
  Try
   txtFileName.PostedFile.SaveAs(strFileNamePath)
  Catch ex As Exception
   Response.Write("Error Saving File <Br> " & ex.Message.ToString)
   Return False
  End Try

 Please help me ! I would really appreciate if you would write me some code, not only explain me !