Some external PDF readers copy highlighted text into the annotation comment field to make it editable. Zotero parses and displays highlighted text itself and uses annotation comments as intended, for editable comments. If you then view annotations created externally within Zotero, you'll see the highlighted text twice. Often, the version of the highlighted text in the comment field will include extra line breaks, since external PDF readers don't always remove them automatically the way Zotero does.

If you're using Zotero to view annotations or add them to notes, you should disable the setting in your external PDF reader that copies highlighted text into comments

In a future version, Zotero will attempt to automatically remove comments with text that duplicates the highlighted text.