Ideally you'd merge rather than delete the duplicate. Zotero remembers the metadata of an item cited in a document even if it's no longer in your library. I think your only option now is to replace all instances of 2021b with the 2021a citation manually
Thanks, I fund an older discussion on this which suggested ensuring that I cite the item in My Library rather than Cited Items. I went back and re inserted this throughout and the problem has been resolved.
I have also made a note to merge rather than delete duplicates. Thanks.
I have also made a note to merge rather than delete duplicates. Thanks.