RANDBETWEEN - maybe not so random

Odds of 1 or more duplicates in 10000 rows : 66.52 %
(The loop calcs the probability of 10000 uniques, which is then subtracted from 1.)

Sub CalcOdds()
    Dim M As Double, i As Double, k As Double
    M = 26 ^ 4 * 10 ^ 2
    k = 1
    For i = 0 To 9999
    k = k * (1 - i / M)
    Next
    MsgBox "Odds of 1 or more duplicates in 10000 rows: " & Format(100 * (1 - k), "0.00") & "%"
End Sub