A few sites have reported an issue where, after upgrading to the latest release of the plugin, the date for an event previously entered into the system will be displayed twice.
This should not be an issue for anyone upgrading to version 3.9.9 from any version before 3.9. However, if you saw this issue crop up after upgrading to one of the earlier 3.9.x releases, updating will not necessarily fix it. Here is a workaround you can use if you have this problem with events currently displayed on your site.
Add the query string ?clean_duplicate_dates=1 to your web domain and load that page. Example:
Then take another look at your calendar to confirm the duplicates are gone.