Late last year, my Duplicator addon for Statamic was merged into Statamic Core.
This meant that if you were using the Duplicator addon previously, you could update Statamic and you'd be able to Duplicate in the same way you could before with the addon.
However, if you're using a multi-site, you'll no longer see the "Duplicate" action show up on the list of available actions for an entry.
This is happening as the Duplicate action was disabled for multi-sites just before it was merged in. If you're interested, there's full reasoning behind it here.
In the meantime, you can re-install my Duplicator addon if you're using a multi-site and you'll be able to duplicate entries just like you are used to:
composer require doublethreedigital/duplicator
You may want to add your 👍 to an open feature request regarding this issue.