When we performing check in to the items already checked in SIP2 do check in of the same instead of blocking as the item is not checked out.
I am not sure this would be the intended behaviour. In the staff interface multiple checkins just generate a note, but you are not blocked from doing it. If the patron checks it in multiple times, it doesn't seem to cause harm. It might also happen accidentally by moving the item in and out of the reader's field.