When you wanttry to raise another flag, after your flag has been declined by a moderator, there will be a warning that reads (I don't remember it exactly):
Your previouslast flag was declined. Please take a look at the declined flag and make sure you raise the right flag - please review (a link for flags for username) it before flagging this time.post!
It happened to me more than a few times and no big deal. There is no negative effect on your reputation and you won't be suspended for flagging an appropriate comment unless you do it for a hobby.
I understand why you are raising this issue, but please be careful when you are flagging. If you are not sure, you don't have to flag it. Let others take care of it. Also, comments are not the important part of Stack Exchange format and they are not designed to last long anyway.
If SE accepts your feature request, I would not object to it strongly, but what if users start to overuse or abuse flagging comments and cancelling them? I think cost outweighs benefit here.
Also, you need to note that questions could be edited for your retraction of a flag, but comments can't be edited after 5 minutes of posting.