You really should never use a duplicate of an existing item to create another, it is technically possible but the issue is you cannot assume any of the specs/data etc. actually match or are correct even though you have what otherwise appears to be the same item with just a different barcode, disc ids in particular are likely to be different.

If you are going to add correct covers with the right barcode and check each and every field is complete and correct you could do it but at that point it would be quicker and easier to just make the new item and likely result in a much more correct data.

If something like the description is an exact match then you can copy and paste that.

As for duplicate barcodes for different titles that is not something users can deal with directly, as it is very rare and would make it way to easy for user to just keep creating new duplicates of existing items you need to follow the procedure here http://kb.mymovies.dk/KnowledgebaseArticle50059.aspx?Keywords=duplicate to request a duplicate be created.

Duplicate barcodes are rare but do happen though I have to say when I see multiple such duplicates like this along with barcode less versions and seemingly made on recordable rather than printed discs I start getting suspicious if these are genuine items or bootleg fakes given a false code to appear more legit. They could be genuine, there are retail items on recordable discs but they are uncommon but seeing DVD 9 DVD 5 which are recordable discs is something of a red flag especially when combined with a duplicate code and what looks like low quality cover printing and some signs of an alteration to the barcode numbering where the printing is not aligned quite right on the numbering, even more so if these came from some random internet store or second hand/marketplace.

I would be suspicious that these are fake recordable copies of legitimate releases using slightly altered or duplicate barcodes to appear less fake but do not have enough info to be sure, though if you got these from a source other than a regular retailer I would say that suspicion rises to a high probability these are bootleg/fake copies.

These may indeed be genuine items, its suspicious but by no means certain and that they were bought from a mainstream retailer would support them being genuine.

Items in box sets can indeed share the same barcode, though those typically are not going to get sold separately as new but also barcodes are not totally unique and duplicates can and do crop up on completely random and unrelated items.

That these items set off a bunch of red flags for fakes may just be a chance coincidence unless googling them turns up anything.

There is not enough of a basis to assume they are fake unless googling shows anything so I can't see a reason to refuse them at this point.