Store: Don't add duplicate results to results.

src/calibre/gui2/store/search.py

@@ -463,6 +463,7 @@ class Matches(QAbstractItemModel):
         self.reset()
 
     def add_result(self, result, store_plugin):
+        if result not in self.all_matches:
             self.layoutAboutToBeChanged.emit()
             self.all_matches.append(result)
             self.search_filter.add_search_result(result)

src/calibre/gui2/store/search_result.py

@@ -22,3 +22,6 @@ class SearchResult(object):
         self.detail_item = ''
         self.drm = None
         self.formats = ''
+
+    def __eq__(self, other):
+        return self.title == other.title and self.author == other.author and self.store_name == other.store_name