double post prevention

Anyone with a crappy internet connection has probably experienced this at least once:
You see post, click "reply", write your content and click "post". Then something goes wrong and you press F5 to try again, telling the browser to send your post again, as it obviously didn't work.
However the server still got the request and created a reply the first time… So now your post is duplicated.

My proposed solution is to: [begin technical stuff]
- create a unique ID when "reply" is clicked and stuff it in a hidden form field.
- when a message is posted check if the ID in the form already exists (from the same user?) and ignore the post if so.
- otherwise post the message as usual and store the ID somewhere.