I also felt I needed this functionality today. I tried to create a folder with all the references from a document to have them in one place and be easy to manage/export to bib. I managed by exporting to bib from the document reference list, importing the bib into a new folder and merging duplicates. It worked well but I had to manually disable a couple of false positive duplicates.
edit: also noticed that merging duplicates changed the bibkey, but after reading this I'm not sure what is the best solution for it