I really try to not write passive aggressive replies but people who don't seem to read before typing are making things harder than they have to be.

"Wow, thanks you so very much for the suggestion. I mentioned that I already tested this scenario and that it seems to be unrelated to this case, but thanks for bringing it up again! Now I really question my ability to get my point across"

This. Makes me want to passive-aggressively hit people.
Add bonus points if it's verbal and the person also includes an explanation.

Like "Oh hey, thanks for explaining to me why I should consider testing $x! While you obviously didn't listen to me when I told you five minutes ago that I tested $x and the result did not indicate that it is relevant, your answer was not only utterly pointless but also succeeded in boring me out with things I already knew."