Resolve "Add confirmation when adding a song to a playlist if it is already in the playlist"
Closes #784 (closed)
Return a validation error on PLT create / playlist add many if the given tracks are already in the target playlist and a allowDuplicate/allowDuplicates flag is not specified in the request body.
Display a message on add-to-playlist / playlist batch add with buttons which allow adding duplicate items to the targeted playlist.
Screenshots (I tweaked with the frontend code in order to generate extra entries for some of these):
(unchanged) Before clicking the 'add' button:
After trying to add a track to a playlist that already contains it. 'Cancel' dismisses the dialog, 'Add anyways' adds the song and dismisses the dialog:
After trying to add songs from the current queue (a few duplicated songs):
After trying to add songs from the current queue (many duplicate songs):
