VBA Generate Randomly No repeating Unique Number

I am working on a excel vba based project where I need to extract random set of data from a given dump. I have prepared the data and it is working fine , Only the problem is sometimes it is selecting the same data twice. I have used the rnd() function for picking the data in random manner but still sometimes it giving same output. Below is the code. Please help me out.

For i = 1 To Worksheets("Output").range("D7").Value - 1
lowest = 1
highest = Worksheets("Output").range("T5")
Randomize
generateno = Int((highest - lowest + 1) * Rnd() + lowest)
Gen = generateno
  Sheets("Workings").Select
        range("B1").Select
        ActiveCell.Offset(Gen, 0).Select
        range(Selection, ActiveCell.Offset(0, 3)).Select
        Selection.Copy
        Sheets("Output").Select
        ActiveCell.Offset(1, 0).Select
        Selection.PasteSpecial Paste:=xlPasteValues, Operation:=xlNone, SkipBlanks _
            :=False, Transpose:=False

    Next i