TYPO3 Core

\TYPO3\CMS\Core\Resource\OnlineMedia\Helpers\AbstractOEmbedHelper::transformMediaIdToFile() starts with a call to $file = $this->findExistingFileByOnlineMediaId($mediaId, $targetFolder, $fileExtension).

When successful, \TYPO3\CMS\Backend\Controller\OnlineMediaController::addMediaFromUrl() does not inform if new media was actually added (= new XYZ.vimeo or XYZ.youtube file) or just existing media found in the folder.

Instead, \TYPO3\CMS\Backend\Controller\OnlineMediaController::mainAction() always renders a flash message saying 'LLL:EXT:lang/Resources/Private/Language/locallang_core.xlf:online_media.new_media.added', which is simply untrue and misleading if the video file was not newly added, but found to exist - possibly under an unexpected file name.

You can choose if the "add media" process is regarded as successful or failed in this case, but nonetheless the user must be warned that no new file was added to the file list.

Target:
Behavior is identical in TYPO3 8 and 9.