Finding duplicates in a column, prompt user then delete the current one

Lifeseeker,
OK, so you manually input data into both sheets and they are not linked. Got that.

I don't know what you want deleted, so for the time being this macro deletes the duplicate you just entered (Tell me if you want it another way?). Enter 123 in cell A3 and see if this is what you are after?

Private Sub Worksheet_Change(ByVal Target As Range)

 Dim ws As Worksheet, ws1 As Worksheet
 Dim Mbox As String, cVal As Range, LR1 As Long
 Dim LR As Long, i As Long, found As Range, cel As Range, rng As Range
 Dim cell_to_test As Range
 
     Set ws1 = Sheets("Main")
     LR1 = ws1.Cells(ws1.Rows.Count, 1).End(xlUp).Row
     
    If Not Intersect(Target, Range("A" & LR1)) Is Nothing Then
          
        Application.ScreenUpdating = 0
        Application.EnableEvents = 0
           Set ws = Sheets("Sub")
           On Error Resume Next
              LR = ws.Cells(ws.Rows.Count, 1).End(xlUp).Row
              Set rng = ws.Range("A2:A" & LR)
              
              Set cVal = ws1.Cells(LR1, 1)
                  For Each cel In rng
                      Set found = rng.Find(what:=cVal, LookIn:=xlValues, LookAt:=xlWhole)
                          If found <> vbNullString Then
                            If Not found Is Nothing Then
                                 Mbox = MsgBox("Duplicate Found, do you want to delete duplicate", vbYesNo, "Duplicate")
                              If Mbox = vbYes Then
                               cVal.ClearContents
                              Else
                               Resume Next
                              End If
                            End If
                          End If
                  Next cel
      End If
          Set found = Nothing
        Application.EnableEvents = 1
        Application.ScreenUpdating = 1
End Sub

Edit: The system is not allowing me to upload a file, paste this in the worksheet module for "Main".
Edit* it worked. See file