I'm also interested in this. After moving my contacts 3 times in a vcs in the ladt yeard it is an absolute mess..

I spend a considerable amount of time trying to find a solution for Nextcloud. I ended up spending an hour deleting and merging contacts.

Does anyone know how I can merge/deduplicate contacts in a .vcf vcard file?

Tonto2 is a python 3/Qt graphical app that runs on desktops. It's main purpose is not to manipulate *.vcf files, but the appendix to the instruction pages tells how, anyway. Tonto2 uses a spread-sheet-like presentation paradigm. With appropriate magical mystical spells, you can import *.vcf as *.csv and sort the *.csv by last-name, phone-number, eMail, zip-code, or whatever. It won't de-dup, but you can spot the duplicates easier once they're collated next to one another in one sequence or another. Show just the significant attributes. Probably you'll want to sort, look, sort, and look again. Killing entries is nearly as simple as checking them off. FAIR WARNING: This process is time consuming, frustrating, and fraught with peril. Keep several versions of your address list until you're sure the final is the one you want to keep forever. My experience is that I always find stuff I want to keep in each of all (sometimes more than two) duplicate entries, so deleting the dup's is not what's called for. Merging means manually copying from one entry and pasting into another. Due to the judgemental nature of how to handle conflicting and out-of-date info, I've hesitated to try to automate the process.