I've seen it happen occasionally. I'm aware of 3 cases, 1 in NA and 2 in EUW, but i'm sure there were more,
Doubles the post is easy. Just check the comment of the red tracker and you will see they're the same.
Missing another one is tricky. I'm sure it happened in NA but i'm not sure when. First problem was that the post was doubled, and i was like, "wtf why is it shown twice", then when i checked a page's entire rioter comments, it had one more, that wasn't shown in red tracker, but should've been iirc chronologically exactly in the place where the double one was placed.
Here's an EUW example (i'm currently checking old posts and since EUW is faster to check i'm checking it first, and this "double" issues happened twice already").
I don't think EUW has a "Discuss the Boards" category so i wanted to place it here, since the same issue is also on NA, and i hope you guys already know about it.