Merging fields of duplicate references


Hope some can help me. I’m using endnote X3 for retreiving full text articles via DOI from within endnote.

I have a 80,000 reference library, which I created over several years by importing references from scopus database. In some references I included the DOI in the downloaded fields, but not in others which was stupid of me !!!. Is it possible to merge the contents of two references, eg I re import some of the references with the DOI fields now, get endnote to identify which are duplicates, then merge the DOI fields of the duplicated references. Unfortunately I can’t just delete the references and re import them with the correct field as I have already made file attachements to full text PDF files(stored locally on my HDD) for over 10,000 of the references without DOI in the field

Thanks for advance for someone with the brains to answer this.