Saving an article with a pdf from Heinonline gets me an article item twice, second item with the pdf attached. Saving a book from worldcat gets me a book item once.
Is this Heinonline specific or did you get this from other sites, too? There might be something off with the Heinonline translator, but before digging in, it'd be good to know if that's a general issue.
Coming here to report the same issue with HeinOnline. Looks like saving using the HeinOnline translator results in duplicate items being added, sans snapshot, but saving via COinS only pulls one instance, with a snapshot. I haven't noticed this anywhere else.
An update to reflect what happens with attachments: When saving using the HeinOnline translator (either by simply clicking the save icon or right-clicking on the save icon and explicitly selecting HeinOnline), two exact duplicate entries are saved, only one of which includes an attachment--the PDF, if auto PDF attachments are enabled in Preferences. When saving using COinS, only one entry is saved, with a snapshot and no PDF.
Although it seems to be happening everywhere in HeinOnline, here's a permalink to one I've tested.
Upgrade Storage
Although it seems to be happening everywhere in HeinOnline, here's a permalink to one I've tested.