You did come across a bit uniformy there Have - but I will ignore that - yes, I meant private messages. I know its the 'post message' button that seems to be having issues, but I've never heard them referred to as 'post messages' and the only pms I've heard of are the private messages where you hit the 'message poster' button. So, to clarify, I received two identical private messages earlier on and I've never had that happen before.
We've started a thread in site stuff so feel free to pop along if you're still having issues. If you could let us know what operating system you're using, which browser and which version then this will help.