VBA generating a random unique alpha-numeric string

I need to create a Unique-ID (string) for each record as I am developing an application which allows users to access a unique URL like:

http://URL.com/BXD31F

The code below works to create the URLIDs:

Public Function getURLID(ID As Double) As String

Randomize
Dim rgch As String
rgch = "23456789ABCDEFGHJKLMNPQRSTUVWXYZ"

Dim i As Long
For i = 1 To 5
    getURLID = getURLID & Mid$(rgch, Int(Rnd() * Len(rgch) + 1), 1)
Next

End Function

How can I ensure that the URLID created is unique? Do I need to query the database to ensure it has not been generated before? The table has 5 million records. A dlookup query would exceed the limitations of my MSAccess database.