2
I do not know how to do it with a querie so I do make something like this:
Instead of limit search for 10 results I limit search for 100 results.
With the Hundred results I make a loop, if the topic is not the same I add the result to a new array, once I have 10 results or hit the 100 results the loop stop. This is how I do it in recent comments hack (or in mycomments module block).
You could take a look to see what I mean.