Why Zotero adds letters like ‘a’ or ‘b’ after the year (and how to fix it)

You cite a source in your paper and suddenly it says:

(Miller, 2001a) and (Miller, 2001b)even though you are absolutely certain that you only used one source by Miller from 2001.

This is not a bug, but rather the expected behaviour of Zotero.

Why this happens

Zotero adds letters such as a, b, cif it assumes that more than one source with identical data (author + year) is used in the document. This so-called 'disambiguation' is an integral part of citing according to citation rules. This is also programmed into the citation style you're using. The most common trigger for this is not the citation style, BUT your library.

The actual cause

In practice, the problem almost always arises as follows:

1. The same source was imported into Zotero several times
2. Both entries were cited in the document
3. Later you realise this and you delete one of the entries in Zotero
4. However, the accompanying citation remains in your Word document

Zotero continues to ‘see’ two different sources in the document – even though only one is visible in the library – and therefore does not delete the 'a' and 'b'.

Why does this not resolve automatically

Zotero citations live inside your document.
Deleting an entry in Zotero does not remove a citation that has already been inserted into a given Word document.

Therefore, it is not sufficient to simply delete duplicates inside your Zotero library.

The only clean solution is

1. Check in Zoterowhether duplicates exist
   → If yes: merge them, don't delete the duplicate!
2. Remove the affected citation completely from the document.
3. Insert new citation → Explicitly select the retained entry
4. Click once on ‘Refresh’.

After that, the 'a' and 'b' should disappear.

Remember

• Always merge duplicates, never delete them
• The disambiguation is 'by design', not a bug
• Already inserted citations must be inserted manually again (Please note that you must use the source cited IN THE DOCUMENT, here marked with orange.)

Once you understand this behaviour, the problem can be quickly resolved – and avoided in future.

And if you want to prevent other common problems, see my article ‘Zotero acting up? How to avoid problems when citing’.