I don’t think I did anything new in the last week or two, but the three duplicate widgets just showed up in the “Sidebar Blog,” and I don’t know how to get rid of them. The are not indicated in the “Sidebar Blog” widget area.
In writing this support question, I figured it out.
I created a “New Sidebar Widget Area” for many pages. A few days ago, I erased the title of a page. I see now that erasing the page’s title has also erased the title from the specific “New Sidebar Widget Area.” The widgets that were assigned to this particular page then also went to the “Sidebar Blog” widget area by default.
I renamed the page, and everything is back to normal.