Sub RndNum()
 Dim valR() As Variant
 Dim i  As Long, c As Long, n As Long
    Randomize
 ReDim valR(1 To 20)
 For i = 1 To 20
      c = 1
    Do Until c = 0
        valR(i) = Int(Rnd * 300 + 1): c = 0
      For n = 1 To i - 1
         If valR(n) = valR(i) Then c = 1
       Next n
    Loop
Next i
 Range("A1:A20") = Application.Transpose(valR)
End Sub