How can i generate random numbers without repeating/duplicate the same number?

What i want it to do is to generate 100 files each file with a different random size. If the line:

long rndsize = rnd.Next(1, 2000);

Generated the number 43 then don't generate this number again.

In the constructor:

CreateFiles(100);

And the method:

for (int i = 0; i < numberoffiles; i++)
{
    Random rnd = new Random();
    long rndsize = rnd.Next(1, 2000);
    FileStream fs = new FileStream(@"c:\temp\files\huge_dummy_file" + i, FileMode.CreateNew);
    fs.Seek(rndsize * 1024, SeekOrigin.Begin);
    fs.WriteByte(0);
    fs.Close();
}