Trying to generate random numbers in VBA, code returning only values that already exist in the table - excel

Question

I cannot get what this is doing.
For a data entry work, I need to generate a transaction number for each entry.
I have a simple function that returns a string created from a random number:
Public Function GetNewTnum() As String
Randomize 10
GetNewTnum = "SL-" & Int(Rnd * 1000000)
End Function
When I call the function in the sub, not only is it generating the same number everytime it runs, the number can already be found in the data set, which only has 577 entries at the moment!
In the sub, after generating the string, it checks to see if has already been used and another function returns true/false. A loop keeps testing until the generated string has not been found and uses this in the entry:
Public Function finddupTnum(ByRef num As String) As Boolean
Dim f As Range
Set f = inputws.Range("tblDataMaster[Tnum]").Find(num, , xlValues, xlWhole)
If f Is Nothing Then
finddupTnum = False
ElseIf Not f Is Nothing Then
finddupTnum = True
End If
End Function
Loop structure works fine. And even before the code reaches the loop, the first random number that gets generated is as described above:
Dim i As Integer
For i = 1 To 7
If Not Me.Controls("Product" & i) = "" Then
With newrow
newtnum = GetNewTnum()
MsgBox "First tnum: " & newtnum
valb = finddupTnum(newtnum)
MsgBox "Found? " & valb
Do While valb = True
newtnum = GetNewTnum()
valb = finddupTnum(newtnum)
MsgBox "New tnum: " & newtnum
MsgBox "New tnum dound? " & valb
Loop
The loop can't end because all the values generated already exist.
Thank you for your help!

Answer 1

In VBA there is the need to use Randomize without a fixed number (the seed). Otherwise it will always generate the same numbers starting from that seed.
https://learn.microsoft.com/de-de/office/vba/language/reference/user-interface-help/randomize-statement

Answer 2

I removed the Randomize statement and now its generating random numbers.
Also, I changed the loop struture. Now instead of finding generating a new random number, it simply adds 1 the first one created, searches and iterates.
I think at least this way, it will never find a duplicate:
newtnum = GetNewTnum()
valb = finddupTnum(newtnum)
Do While valb = True
newtnum = newtnum + 1
valb = finddupTnum(newtnum)
Loop
And changed the function to make it the right data type ;)

Answer 3

This should work replacing your two functions - first to get random number, then check the current column for duplicates
Option Explicit
Const START_NUM As Long = 1
Const END_NUM As Long = 1000000
Const RANGE_COL As String = "A"
Const RANGE_NUMS As String = RANGE_COL & START_NUM & ":" & RANGE_COL & END_NUM
Public Function GetRandomNumber()
Dim NewNum As Double
Dim NumEntries As Long
Dim NumTries As Long
NewNum = Application.WorksheetFunction.RandBetween(START_NUM, END_NUM)
NumEntries = Application.WorksheetFunction.CountA(Range(RANGE_NUMS))
NumTries = 1
While (Application.WorksheetFunction.CountIf(Range(RANGE_NUMS), NewNum) > 1) And (NumTries <= NumEntries)
NewNum = Application.WorksheetFunction.RandBetween(START_NUM, END_NUM)
DoEvents
NumTries = NumTries + 1
Wend
Debug.Print NewNum
End Function