References from the same author and year should be labeled a, b, ...

parrenin

Using the Climate of the Past style, if two references have the same first author and year, they are not distinguished in zotero, e.g. it gives something like:

Gagliardini et al., 2007, 2007

One would expect:

Gagliardini et al., 2007a, 2007b

Gracile

Works for me. Styles should update automatically but you can reinstall it from the repo to make sure you've got the latest version (update "Copernicus publications" too).

Then, in your document, switch to another style and switch back to "Climate of the Paste" (Zotero Set Docs Prefs button).

parrenin

Actually, it was really the same reference used twice, hence this behavior.
So this is not a zotero bug.