It has happened to me ever since I started using it in January. When the problem happens, you need to close the browser tab hosting HG and open it back up from scratch. You will then get a single prompt.
For me, this problem is triggered when I leave the tab open, then come back to it the next day after using my phone for other things. If I have time, I may try some troubleshooting to see how it can be resolved on the HG side. Essentially, we want to show a single login prompt, and halt ALL http requests until login occurs. I think this may need to be handled on the client side in javascript, as it may have something to do with queued up http requests from the chrome browser as it goes to background.
FYI: I am using latest Chrome build on my Samsung Galaxy S4.
-A