TYPO3 Core

Tested with TYPO3 9.5.8 and the "Form" Content Element.

• Throttle your network to a slower connection. (Like 3G in Chromes Performance panel)
• Go to the Pages module and use the New Content Element wizard to start creating a new "Form" Content Element.
• Do not save the record yet. Switch to the tab "Plugin" and choose a form definition.
• TYPO3 will now ask for a confirmation to refresh the form. Click "OK".
• Now IMMEDIATELY click "Save".

If the connection was slow enough, there should now be 2 identical content elements saved on the page.
This apparently happens, because TYPO3 will automatically save a content element when using a onChange="reload" select field. But because of the slow connection, TYPO3 hasn't finished the ajax request for saving the record in the background yet and when the editor immediately clicked "Save", a second ajax request for creating a new record was triggered.