Tested and working: 
Option Explicit
Sub find_dups()
    ' Create and set variable for referencing workbook
    Dim wb As Workbook
    Set wb = ThisWorkbook
    ' Create and set variable for referencing worksheet
    Dim ws As Worksheet
    Set ws = wb.Worksheets("Data")
    ' Find current last rows
    ' For this example, the data is in column A and the duplicates are in column C
    Dim lngLastRowData As Long
    lngLastRowData = ws.Range("a1048576").End(xlUp).Row
    Dim lngLastRowDups As Long
    lngLastRowDups = ws.Range("c1048576").End(xlUp).Row
    ' Create and set a variable for referencing data range
    Dim rngData As Range
    Set rngData = ws.Range("a2:a" & lngLastRowData)
    Dim lngRowCount As Long
    lngRowCount = 0
    Dim clData As Variant
    Dim lngCount As Long
    Dim lngRowIndexData As Long
    Dim lngRowIndexDups As Long
    lngRowIndexDups = lngLastRowDups + 1
    ' Variable to store those values we've already checked
    Dim strAlreadySearched As String
    For Each clData In rngData.Cells
        ' Reset variables
        lngCount = 0
        ' See if we've already searched this value
        If InStr(1, strAlreadySearched, "|" & clData.Value & "|") = 0 Then
            ' We haven't, so proceed to compare to each row
            For lngRowIndexData = 1 To lngLastRowData
                ' If we have a match, count it
                If rngData.Cells(lngRowIndexData, 1).Value = clData.Value Then
                    lngCount = lngCount + 1
                End If
            Next lngRowIndexData
            ' If more than 1 instance
            If lngCount > 1 Then
                ' Dup's were found, fill in values under duplicates
                ws.Cells(lngRowIndexDups, 3).Value = clData.Value
                ws.Cells(lngRowIndexDups, 4).Value = lngCount
                ' Drop down a row
                lngRowIndexDups = lngRowIndexDups + 1
                ' Capture this value so we don't search it again
                strAlreadySearched = strAlreadySearched & "|" & clData.Value & "|"
            End If
        End If
    Next clData
End Sub