One way:
Run this macro.
I assumed your part numbers are in Column A in both sheets and you want the
numbers in the third sheet to be in Column A.  I also assumed your sheets
are named "One", "Two", and "Three".  Change these as necessary.  HTH  Otto
Sub CopyDups()
     Dim rColAOne As Range
     Dim rColATwo As Range
     Dim Dest As Range
     Dim i As Range
     Sheets("One").Select
     Set rColAOne = Range("A2", Range("A" & Rows.Count).End(xlUp))
     With Sheets("Two")
           Set rColATwo = .Range("A2", .Range("A" & Rows.Count).End(xlUp))
     End With
     Set Dest = Sheets("Three").Range("A2")
     For Each i In rColAOne
           If Not rColATwo.Find(What:=i.Value, LookAt:=xlWhole) Is Nothing
Then
                 i.Copy Dest
                 Set Dest = Dest.Offset(1)
           End If
     Next i
End Sub