This is the first comments.
There is a bug with Block system on 2.1.0,maybe.
I did the operation such as follows.
1.Make topics like this.
topic_A--|
topic_A_1
topic_A_2
2.Make a block to show on topic_A_1 and topic_A_2.
ex) The Block ID is "block-ab"
Then I open the topic_A, there are exactly the same two "block-ab" on the sideblock.
http://demo.geeklog.jp/index.php?topic=test
Would you check it?