I think I’ve found a workaround. In the Fossify Contacts app on my phone, in the settings I checked the box marked ‘merge duplicate contacts’. I then used the AOSP Contacts app to export them as a .vcf which I imported into Thunderbird, which is now showing no duplicates. I don’t know why the AOSP app is able to export without duplicates, even though it’s the Fossify app which has the ‘merge duplicate contacts’ option but there you have it…