Do not push status to feed if its reblog is already inserted (#6488)

A complemental change for precompute_feed_service_spec.rb also fixes its
random failure which is caused by the Snowlake randomization of the order
of an original status and its reblog.

app/lib/feed_manager.rb

@@ -220,6 +220,14 @@ class FeedManager
         return false
       end
     else
+      # A reblog may reach earlier than the original status because of the
+      # delay of the worker deliverying the original status, the late addition
+      # by merging timelines, and other reasons.
+      # If such a reblog already exists, just do not re-insert it into the feed.
+      rank = redis.zrevrank(reblog_key, status.id)
+
+      return false unless rank.nil?
+
       redis.zadd(timeline_key, status.id, status.id)
     end