see if something like this is faster
Option Explicit
Sub DupFinder()
Dim rData As Range, rArea As Range, rCell As Range
Dim aryData() As Variant, aryAddr() As String
Dim i As Long
Application.ScreenUpdating = False
Set rData = Range("C5:G23").SpecialCells(xlCellTypeConstants)
rData.ClearFormats
ReDim aryData(1 To rData.Cells.Count)
ReDim aryAddr(1 To rData.Cells.Count)
i = 1
For Each rArea In rData.Areas
For Each rCell In rArea.Cells
aryData(i) = rCell.Value
aryAddr(i) = rCell.Address
i = i + 1
Next
Next
For i = 1 To rData.Cells.Count
If Application.Count(Application.Match(aryData, Array(aryData(i)), 0)) > 1 Then
Range(aryAddr(i)).Interior.Color = vbRed
End If
Next i
Application.ScreenUpdating = True
End Sub