Prevention of Duplicate entries in playlist

[umunais]

Please add an option to not automatically add a particular video link again and again to a playlist of its already there <!--
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[wb9688]

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[opusforlife2]

Before even that, first check if the issue already exists before opening... although in this case it is understandable if you didn't find anything because #4186 doesn't use the word 'duplicate' even once. O.o