So while I farming the forge master achievement I was screwing around with collectors while waiting for my forge to build and all of a sudden I noticed that I had built two forges. I have tried replicating the bug, but I can't seem to find out what triggered it. Here is proof http://steamcommunity.com/sharedfiles/filedetails/?id=494470770
It's probably a bug where I think 1 of your collectors got confused about which picture to take and took a forge picture.
If it was not functioning like the normal forge, then it's the collector one. If it is and you have 2 working forges, then... um... congrats! ;D But then again I might be wrong on both of my points. :P
Hope that answered your question.