Nevertheless, I can think of a few drawbacks too for visually adding a marker to the reported posts
The only possible way of letting someone know if the post is reported or not without the drawbacks mentioned is to add a list of reports already made for that particular post under the comment section of reporting page. The list should contain all the reports that are submitted for that post along with the reporters username and the comment (that would demonstrate the reason for that report).
After pressing the "report to moderator" button, you would be able to see if the post has already been reported or not, and if reported, for what has it been reported. And then if you think that the reason it has been previously reported for is different from yours, you can go ahead and submit your report as well, but if you find the reason to be the same, you can simply skip it and move forward.