VBA-duplicate value (not using DV)

first time post, long time stack surfer. I have a question regarding catching a user for entering dupe values in a sheet. We are unable to use data validation because cut/copy/paste throws out the data validation and allows them to enter the dupe value. I was originally using this code:

Option Explicit

Public Sub Workbook_SheetChange(ByVal Sh As Object, ByVal Target As Range)

'******problem when copying entire row and pasting into new row, enables user to paste dupe Box ID #******

'Defining variables in Mailroom
Dim WS As Worksheet, EvalRange As Range

'Range to check for duplicates
Set EvalRange = Worksheets("Mailroom").Range("Box_ID_Number")

'Checking if entered value is in the defined range; also if cell is empty exit macro
If Intersect(Target, EvalRange) Is Nothing Or Target.Cells.Count > 1 Then Exit Sub
If IsEmpty(Target) Then Exit Sub

'If user enters dupe value in specified range then error message pops up and event is undone
If WorksheetFunction.CountIf(EvalRange, Target.Value) > 1 Then
    MsgBox Target.Value & " already appears as a Box ID Number. Please enter a unique ID."
    Application.EnableEvents = False
    Application.Undo
    Application.EnableEvents = True
End If

End Sub

The code works fine for keeping a user from typing a dupe value in the column for "Box ID Number." The problem I am having is that if the user is to copy a Box ID Number from a column and another cell from a different column, they are able to paste a Dupe Value that the _SheetChange does not catch. When we were first creating the code for this we were disabling cut/copy/paste functions; however, others using the sheet apparently still need that function for other portions of the sheet.

Any ideas?