AAARRRGGGHH!!!

Sorry, but this is EXACTLY the same problem I'm having. DNN 4.9.5 - Every solution thus far is failing. Here's what I've noticed:

It seems to ONLY affect the HTMLModule, and just as Amirb said, it specifically adds another HTMLModule prefix to the module string (i.e.dnn_ctr1234_HTMLModule_HTMLModule_lblContent)

I initially thought this might have occurred when we changed the expires headers for the affected install through IIS, though we also modified the performance settings at the Host level right around the same time, so it's possible that it could have been something I did there - I doubt it, I honestly think this is a bug somewhere. We also set expires headers on a DNN 5 install and tweaked the performance on there as well, with none of these side effects. 

Upgrading to DNN 5 isn't an option for us immediately, so I'd love to be able to determine a solution for this.

We've changed the module cache settings manually, retested (after clearing the cache) and everything will seem ok - but then the next day we'll have duplicated content again. We've disabled expires headers for this install through IIS though it doesn't seem to have had any effect. I'm definitely seeing a negative consequence in all the SERPs because of this.  Google, Yahoo and Bing are all seeing this, yet the normal user doesn't. It's just whacked.

I've considered opening up the web.config file and just flooding it with curse words so this install knows how I feel, but something tells me that wouldn't be very productive. Has ANYONE solved this conundrum?