Show when adding duplicate rss feeds

[hollow-owl]

Is your feature request related to a problem? Please describe.
Currently there is no indication when adding a feed if it is a duplicate. The "+Add Livemark" button will add a new entry even when the same rss feed is already there. Generally I would not want to add a feed multiple times on purpose.

Describe the solution you'd like
The "+Add Livemark" button should change to indicate that the feed is a duplicate. It could become greyed out and say "duplicate" or something similar.

Describe alternatives you've considered
Looking through a list of existing feeds to make sure that I'm not adding a duplicate

Additional context
Example showing multiple folders for the same feed.