Check Duplicates - Speed it up.

I had a requirement to check for the presence of duplicates in a column in Excel.
I have written a function for this purpose and it works. The drawback is that it is an extremely slow function and takes about 6 minutes to check across 20000 rows. Could someone take a look and speed up the function?

VB Code:

Sub CheckDuplicate()
    Dim txtSearchVal As String
    Dim lCtr As Long
    Dim lCtr2 As Long
    Dim bFound As Boolean
    
For lCtr2 = 1 To 20000
    txtSearchVal = Sheet1.Cells(lCtr2, 1)
    For lCtr = (lCtr2 + 1) To 20000
        If Sheet1.Cells(lCtr, 1) = txtSearchVal Then
            Sheet1.Cells(lCtr, 1).Font.Color = vbRed
            Sheet1.Cells(lCtr, 1).Font.Bold = True
            'Sheet1.Cells(lCtr, 1).AddComment ("Duplicate Values")
            Exit For
        End If
    Next lCtr
Next lCtr2
End Sub

Do you have data all the way to the 20000th row?

Yes. We do.

I have used this page a few times to straighten me out in my coding endeavors:
http://www.ozgrid.com/VBA/VBALoops.htm

Here is what I think might interest you, modified slightly to your needs:

VB Code:

Sub Dupes()
Dim DupeValue As Double
Dim ii As Integer
DupeValue = 2 'whatever
    If WorksheetFunction.CountIf(Cells, DupeValue) = 0 Then
       MsgBox "No duplicate values."
       Exit Sub
    End If
    Range("A1").Select
    For ii = 1 To WorksheetFunction.CountIf(Cells, DupeValue)
        Cells.Find(What:=2, After:=ActiveCell, LookIn:=xlFormulas, _
        LookAt:=xlPart, SearchOrder:=xlByRows, _
        SearchDirection:=xlNext, MatchCase:=False).Activate
        ActiveCell.Interior.ColorIndex = 4 'green highlight
    Next ii
End Sub

This still uses a loop, and perhaps someone else knows the way to get the cells.find function to find ALL the values at once, and not just the first one. I looked, but I didn't find any resources regarding that. :(
This loop's running time will directly depend on the number of duplicates in the sheet. If its only a few hundred, it shouldn't take too long to run at all, even on this old Pentium 166.

Hope this helps you out!