sCells will never be empty since it is initialized as a null string. But
you can do

Sub Dups()
Dim iLastRow As Long
Dim i As Long
Dim sCells As String
Dim rng As Range

iLastRow = Cells(1499, "B").End(xlUp).Row 'Cells(Rows.Count, "B")
Set rng = Range("D1:D" & iLastRow)
For i = 1 To iLastRow
If Application.CountIf(rng, Cells(i, "D")) 1 Then
sCells = sCells & Cells(i, "D").Address(False, False) & ","
End If
Next i
If sCells < "" Then
sCells = Left(sCells, Len(sCells) - 1)
MsgBox "Duplicates found in " & vbCrLf & sCells
Else
MainUpdate
End If
End Sub

--
Regards,
Tom Ogilvy

"Steph" wrote in message
...
Hi. The code below is compliments of Bob Phillips. It scans the contents
of column D, and finds duplicates. If the code finds them, a message box
displaying where the dulicates are found is displayed. This code works
great if there are duplicates found. If there are no duplicates, I get an
Invalid procedure call or argument error, with the line sCells =
Left(sCells, Len(sCells) - 1) highlighted by the debugger. Any ideas how
to fix? Thank you!!

Sub Dups()
Dim iLastRow As Long
Dim i As Long
Dim sCells As String
Dim rng As Range

iLastRow = Cells(1499, "B").End(xlUp).Row 'Cells(Rows.Count, "B")
Set rng = Range("D1:D" & iLastRow)
For i = 1 To iLastRow
If Application.CountIf(rng, Cells(i, "D")) 1 Then
sCells = sCells & Cells(i, "D").Address(False, False) & ","
End If
Next i
sCells = Left(sCells, Len(sCells) - 1)
If Not IsEmpty(sCells) Then
MsgBox "Duplicates found in " & vbCrLf & sCells
Else
MainUpdate
End If
End Sub