Any place is fine to ask questions... users have been good about forwarding us to where the conversation is happening.
Tracking down info on the Duplicate Messge error is tricky: http://stackoverflow.com/questions/4874532/twitter-error-could-not-post-tweet
I have not found it spelled out in the Twitter API, but it does exist.
For example, if you send, "The temperature is 56" and "The temperature is 60" one message will likely get disregarded. You can even add time stamps or random numbers, and will still bounce - they have some matching detection. You are safe, I think every 3-4 hours if the content is the same.
Wow, on the one hand I get suppressing spam, but as far as the IOT, that is an exceedingly bad idea on Twitter's part, automated monitoring messages will by definition tend to be very similar! Maybe it should be a user-configurable parameter, whether to filter or not. I'd even be OK with it being on by default.
If you keep the Serial Monitor window open on the Arduino sketch, you will see what gets reported back. That's where you might see Bad Gateway or a 403 error.
Yes, I am quite aware, like I said, I was seeing the 502 error simultaneously
in my web browser, and
being returned from the server in response to POSTs. The examples I posted in the ThingSpeak forum were copied from the serial monitor. BTW, I wrote my own sketch, which is why I asked for a pointer to the one you referenced that would reproduce the 502 error. Not sure mine is experiencing that.