Support multiple entries in a playlist for the same video

[dennis531]

The plugin currently only supports one entry for one video per playlist. This restriction is implemented in the database. On the other hand, Opencast playlists can contain multiple entires of the same video in one playlist. As we do not have a specific use case for this, we should not provide this function in the UI. Nevertheless, the plugin should be able to handle multiple entries of the same video in the database and display them to the user in order to avoid incompatibilities.

Related #955

[dennis531]

I think this shouldn't be a problem or necessary at all. In

studip-opencast-plugin/lib/Models/Playlists.php

Lines 616 to 624 in 2c1e834

	$playlist_video = PlaylistVideos::findOneBySQL('video_id = ? AND playlist_id = ?', [$db_video->id, $this->id]);
	if (is_null($playlist_video)) {
	$playlist_video = PlaylistVideos::create([
	'video_id' => $db_video->id,
	'playlist_id' => $this->id,
	'service_entry_id' => $entry->id ?? null,
	]);
	}

multiple entries of the same video are ignored. Still, if the user saves the playlist, the multiple entries will be lost.