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 I recently ran into an issue where my CalDAV provider went down for a couple of hours so many synced events were displayed as duplicates in my macOS Calendar app. The CalDAV account was configured via a <a href="https://developer.apple.com/documentation/devicemanagement/profile">Profile</a> so removing it meant I also needed to re-configure my mail account.
 <h2>Reset Calendar.app</h2>
 Fortunately there's a way to reset all synced events without loosing any preferences.
 <ol>
 <li>Quit the Calendar app</li>
 <li>In Finder: Go to Users ▸ You Profile ▸ Library ▸ Calendars
 <ul>
 <li>If you don't see the Library folder, press CMD ⌘ + Shift ⇧ + . (Dot)</li>
 </ul>
 </li>
 <li>Delete the following files:
 <ul>
 <li><code>Calendar.sqlitedb</code></li>
 <li><code>Calendar.sqlitedb-shm</code></li>
 <li><code>Calendar.sqlitedb-wal</code></li>
 </ul>
 </li>
 </ol>
 Now, re-open the Calendar app, press CMD ⌘ + R and wait for the calendars &#x26; events to re-sync cleanly. This can take a couple of minutes based on the size of your remote calendars.
 If you're missing any items in the sidebar, go to Settings ▸ Accounts and toggle "Enable the account".
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Reset Calendar App on macOS

Reset Calendar.app