The heart of the issue is forumers clicking the post button multiple times. My personal solution: Whenever the website hesitates to post my post, I highlight and copy my content, refresh the page, check to ensure that the forums haven't already registered my post, paste, and post.
I'm not sure how intricate it would be to code your suggested feature, but I personally don't think double posts are that big of an issue. Not enough of an issue to take web development time. I'd rather they dedicate that time to new forum features. This, of course, is based on the assumption that it would take a good deal of work to implement your suggestion.
Jagex could introduce a 1 minute anti-spam feature, so you're only permitted to make one post every minute.
But as Jeremy says, highlight your text and copy it to your clipboard instead of just clicking again and again until the page loads. However I have made a few double posts myself recently because my mouse sometimes get dirt inside that causes single clicks to register as double clicks, it also makes it very difficult to drag items around in my inventory etc. The only thing preventing that would be an anti-spam feature, which would really be helpful. But it would be detrimental to community threads that would take a person at least 30 minutes to create a thread with the first 3 pages reserved for community rules and information.