With the help of the comments, and having done some more research, I can provide a partial answer (I am happy to update it as others comment on it). The main strategy to address this problem has two parts.
1) Identify the repeated entries and merge them.
There are a number of tools and scripts out there that can be used to identify duplicate entries in a .bib file. From all of them JabRef seems to be the best one.
2) Create an alias for the two keys that point to the same reference so that both coauthors can use their own key.
- If you use Biblatex the solution is very simple: under the entry with the key of the entry you are keeping you add the field IDS with the alternate key (i.e.,
IDS = {secondKey}) (See the selected answer in this Stack Exchange thread for more info)
- If you use plain BibTex, you need to define a new citation command (
\acite) and a \bibalias[1] command that allows you to cite the same entry with two different keys (see here for how to define the new citation command and the alias)[2].
(Incidentally: if you want to share with your coauthor only the references cited in the paper (instead of your entire .bib file) use bibexport. This utility allows you to create a BibTeX file only with the entries that are cited in the document you are coauthoring).
[1]: Don't confuse \bibalias with natbib's \citealias. The latter allows you to cite a reference with a special designation (different from the designation automatically provided by natbib).
[2]: This strategy has been formalized in the bibalias package (available here) Among its drawbacks is that it only works for the \cite command. If anyone knows of a solution to adapt \citep and \citet that would be great.
author,title,year, etc fields. What's the upshot? Your co-author and you will (a) have to merge your respective bib files and, likely, apply some sorting and (b) manually go through the merged file and detect which entries have different keys but are duplicates in all other respects. (Software tools do exist to help with merging and sorting bib files.) For any near-duplicates, you'll also have to decide whose key to retain. How do you and your coauthor manage your bib files? – Mico Feb 07 '20 at 05:39