For the block of CBB, if I have a member replied to a post, it will show RE:...

For example:

If the original post is
"Question about something"

and a replied to this post will show
"RE: Question about something"

If there are 10 replies, my last 10 post will all be the same:
"RE: Question about something"
"RE: Question about something"
"RE: Question about something"
"RE: Question about something"
"RE: Question about something"
....

Is there a way to group the topic up to a single post in the block?

Something like:
GROUP BY p.topic_id

Please help, thank you.

Look for a block called "Recent Topics" in the list of blocks generated by CBB/Forum, I think that's what you want.

;o)

I think I should add "GROUP BY p.topic_id" to newbb/blocks/newwbb_block.php

But I tried, failed.

What is the query, you want to modify?
If you use the group by clause, you can only have the fields from the group by clause or aggregate functions on the other fields in your value list to retrieve.