Fix #4470: Check URLs for duplicates when creating artists

Show a warning when creating a duplicate artist; that is, when adding a
URL that already belongs to another artist.

This is a soft warning rather than a hard error because there are some
cases where multiple artists legitimately share the same site or account.

app/models/artist_url.rb

@@ -5,6 +5,7 @@ class ArtistURL < ApplicationRecord
 
   validates :url, presence: true, uniqueness: { scope: :artist_id }
   validate :validate_url_format
+  validate :validate_url_is_not_duplicate
   belongs_to :artist, :touch => true
 
   scope :url_matches, ->(url) { url_attribute_matches(:url, url) }
@@ -140,6 +141,14 @@ class ArtistURL < ApplicationRecord
     errors.add(:url, "'#{uri}' is malformed: #{e}")
   end
 
+  def validate_url_is_not_duplicate
+    artists = ArtistFinder.find_artists(url).without(artist)
+
+    artists.each do |a|
+      warnings.add(:base, "Duplicate of [[#{a.name}]]")
+    end
+  end
+
   def self.available_includes
     [:artist]
   end