I think sometimes the "this has been discussed a million times before" comment is more to let the poster know that perhaps they won't get any/many responses.
After I've answered the same question several times, it becomes easier to just point to a prior thread on the topic.
The other approach would be to simply ignore and not respond AT ALL to those repeat questions.
You know, "if you can't say something nice, don't say anything at all." That's helpful.