Sometimes a lot of excess notifications might appear on your Mac (often effectively the same one hundreds of times). Closing them one at a time can take a long time; sometimes they seem to go on forever. A restart usually fixes this, but you can also fix it with one simple terminal command...
Run the "Terminal" app (it is in the "Utilities" folder, in the "Applications" folder). Type: "killall NotificationCenter" (without the quotes). The notifications should disappear. Quit from Terminal.
The process starts itself again quickly, so real notifications should start appearing.
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