Quick find code: 409-410-882-66109064
From what I can tell, you're pressing the post button, the server is getting your request and adding it to the thread, but it's then crapping out in the response, so your browser never renders it, so you think it hasn't worked and press the button again(?).
It suggests a browser or network issue. Have you tried resetting your browser entirely to default configuration? Or, using another browser to see if you get the same issue? If you get the same issue on a fresh browser, that suggests a connectivity problem to me. Probably something like high packet loss or unreliable connection, such that it would seem to mostly be working, but just enough to bugger things up.
05-Jul-2019 12:33:22 - Last edited on 05-Jul-2019 12:33:56 by Moneybucks
You can get around that if you configure your device and/or gateway to use an alternative DNS server. I use Google's free public DNS service, which hasn't let me down - if your DNS is crap, and you're using whatever counts as a default, then try Google DNS.
Primary DNS: 184.108.40.206
Secondary DNS: 220.127.116.11
You can find those settings somewhere in your router/gateway (usually in networking), or to configure it just on one endpoint, then look in the networking settings.
For Windows (I'm making an assumption), take a look at the below.
Of course, if it's just that your network is unreliable, this may not help you - as the DNS resolution is one of the first flows to go over the network, if your network is gone, then it can come back as a failed DNS lookup just because it timed out.
05-Jul-2019 14:41:32 - Last edited on 05-Jul-2019 14:42:46 by Moneybucks
It could be a timeout, in that the server made no response. But "Empty" response suggests to me that there was a response, just with no content. That would support my earlier hunch that it was an unreliable network connection.. In that case, you might find that some packets are received, but others aren't, so you'd not get a timeout, but neither would you necessarily get anything back that could be rendered, but that's my hypothesising. Truth is I don't know the significance of that error on Chrome.
Is it just the Runescape website this is happening on? A generally unreliable network should mean you see this in other places too.
I did find one useful bit of information in my Googling, which would make some sense in this case, as your DNS has been changed. Try flushing your DNS resolver cache, and see if that helps at all.
Run the following on a command prompt
> ipconfig /flushdns
The Chrome error your getting later is saying that Chrome attempts to contact the server but the connection is closed before anything is actually sent through it. It's entirely distinct from a general fault like page not found or some such, because although those are errors, they're still communicated from the server to your PC. The actual error you're getting is your PC makes a successful connection, but then it is closed before any information is transferred over it.
As to how it relates to the main issue of double posts, it definitely seems like the connection you're using is unstable. VPN's, antiviruses, bad wifi, and anything in general that might interfer with the internet on your PC could be a potential issue here, if not the internet provider you're using is the problem directly.
It's honestly a pretty vague thing to diagnose, the only thing I have to contribute is I don't think it's a RuneScape specific problem, just that the RS forums are making it more evident than other software might because they don't have any features designed to prevent accidental multiple submissions.
I'd run a network connection test like the following and see if there's anything dodgy with your network connection in general, particularly, packet loss.
06-Jul-2019 01:14:14 - Last edited on 06-Jul-2019 01:24:39 by Hmm
I'd start considering physical things in your house such as any wires that might be leaking interference near your router or antenna, or the quality of any ethernet connections and whatever you're using. It definitely sounds like something's dodgy at the physical level somewhere down the line, though it could still be the ISP's fault there.