Check Duplicates - Speed it up.

[abhijit]

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?

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