Why did I get a duplicated Works Cited page in my Google Doc?

The Scrible Writer Google Docs add-on tracks where the Works Cited page content is by use of some special characters before and after the bibliography content.  If you accidentally remove any of these while editing the document, the add-on won't find the existing content and will instead append a new Works Cited page.

You should generally avoid editing in the Works Cited section (other than the title if the default is not appropriate for your particular case) since this is overwritten any time you add a new quote or refresh to pick up changes to the inline citations.