Detect existing URLs on feed redirection (bug 1457)

When the feed is redirected, we usually simply rewrite
the URL. However, if the new URL already exists in the
subscriptions, we must not allow this, and ideally
fail to update the feed to avoid duplicates.

src/gpodder/gtkui/main.py

@@ -2093,14 +2093,16 @@ def add_podcast_list(self, urls, auth_tokens=None):
         if auth_tokens is None:
             auth_tokens = {}
 
+        existing_urls = set(podcast.url for podcast in self.channels)
+
         # Sort and split the URL list into five buckets
         queued, failed, existing, worked, authreq = [], [], [], [], []
         for input_url in urls:
             url = util.normalize_feed_url(input_url)
             if url is None:
                 # Fail this one because the URL is not valid
                 failed.append(input_url)
-            elif self.podcast_list_model.get_filter_path_from_url(url) is not None:
+            elif url in existing_urls:
                 # A podcast already exists in the list for this URL
                 existing.append(url)
             else:

src/gpodder/model.py

@@ -1091,6 +1091,8 @@ def update(self, max_episodes=0, mimetype_prefs=''):
         except feedcore.NewLocation, updated:
             feed = updated.data
             logger.info('New feed location: %s => %s', self.url, feed.href)
+            if feed.href in set(x.url for x in self.model.get_podcasts()):
+                raise Exception('Already subscribed to ' + feed.href)
             self.url = feed.href
             self._consume_updated_feed(feed, max_episodes, mimetype_prefs)
             self._update_etag_modified(feed)