You know, we could all say "no duh" to "no duh" since when things are obvious it's pretty obvious they're obvious so you can be sarcastic to the person saying it's obvious because we all know that it is obviously obvious. I mean, obviously, amirite?

I had to look at it for awhile but I get it and I'm gonna attempt to explain:
When people say "No duh" they're saying that something is obvious. OP is saying that it's obvious when things are obvious, so the person who pointed out that something was obvious with a "No duh" deserves a "No duh" in return. Which means that you can pretty much get caught in an endless loop of "No duh"s. Did that resemble something that made any sense?