If you're going to use the form's Error event, you need to interrogate
the DataErr argument that is passed to the event procedure:

   Private Sub Form_Error(DataErr As Integer, Response As Integer)

       If DataErr = 10014 Then
           Response = acDataErrContinue
           MsgBox _
               "These samples may have already been entered " & _
               "in the database. Select another disk or " & _
               "click yes to continue"
           Me.Undo
       End If

   End Sub

But it would be better to trap the error in the procedure that raises
it.  So if you have a procedure that currently looks roughly like this
...

   Private Sub cmdAppend_Click()

       ' ... some code ...

       CurrentDb.Execute "Your Append Query", dbFailOnError
       ' or DoCmd.RunSQL, or DoCmd.OpenQuery

       ' ... more code ...

   End Sub

... then you would do better to establish error-handling in that
procedure.  In that case, you *would* refer to Err.Number to identify
the error.  It would look something like this:

   Private Sub cmdAppend_Click()

       On Error GoTo Err_Handler

       ' ... some code ...

       CurrentDb.Execute "Your Append Query", dbFailOnError

       ' ... more code ...

   Exit_Point:
       Exit Sub

   Err_Handler:
       If Err.Number = 10014 Then
           MsgBox _
               "These samples may have already been entered " & _
               "in the database. Select another disk or " & _
               "click yes to continue"
           Me.Undo
       End If
       Resume Exit_Point

   End Sub