I'm not sure if this is a common thing or not, but I have come across it many times. Once in a while, when i am posting on another players wall, When i submit the post it repeats the person who posted last on that the wall. It's not a big deal, I'm just putting it out there for the AG team to see, and be aware of.
When you post the comment, does your comment show up? If not, then you commented or posted somewhere else beforehand and you didn't wait 30 seconds before posting/commenting again.
You simply have to wait 30 seconds before posting again. I think it would be nice if they actually told you instead of just having a submit button (like they have in the forums). I think its freaked everyone out at some point in time.