At Longleat we have a relatively simple catalogue compared with that of a larger institution. We do often have duplicate items though and our practice has always been to create new bibliographic records for each item. This makes every record tied to a unique copy.
The only exceptions to this have been where one item is split between more than one book. Then two copies and two shelfmarks and two binding descriptions will have to be added to the one record. This does crop up from time to time. Also, when there are two copies bound together (usually pamphlets) then only one record is created and a 500 note is put in to the effect of 'another copy at same location'.