Dicecoin.io redirects to duckdice.io for certain when I go there. It's because I'm going to it from a US IP and if I use a VPN I can actually get to dicecoin.io. Cryptogames.io also blocks USA users although that was a later in the history of the site when they started doing that.
USA is blocked by many dice sites but are you sure dicecoin redirects to duckdice if we visit from US IP? This sound quite serious now, if all three of this sites are connected than duckdice might also loss all the reputation they have earned so far. I have also played there and really like the user interface and support duckdice have. 
Yes. I'm 100% it redirects to duckdice. I have ever actually seen the dicecoin.io page until you mentioned it not redirecting for you.
Tested (from usa):
It's a referral link, but not sure why would they redirect to them. It's indeed a possibility that cryptogames.io = duckdice.io = dicecoin.io