It is important that you do not save the same source multiple times in Zotero. Then you may end up with small differences in your references, and that the same source is listed several times in your bibliography.

If you have multiple entries from the same source in Zotero, you must delete or merge them.

You can:

Right-click on the source you want to delete and select Move Item to Trash

Image that shows how to delete a citation.

Or:

Go to the Duplicate Items folder located on the left side of your library, under My Library.

Here, an automatic overview of similar sources is created. You can then click on one of the sources and both (possibly more) will be highlighted.

If this is the same source and you want to put these two together, select Merge 2 items on the right side. The sources will then be converted into one.

We recommend that you check the source after merging it to make sure the information is correct.

Image that shows how to merge two citations into one.