Two overwintered queens

I went to check my 24 hour old grafts today to find 1 out of 42 accepted. I looked closer and I found new eggs in my cell builder then there she was. When I made my cell builder I found one queen and placed her on a frame and put it aside. I assumed she was the only one. then I shook bees into the top super with a cloake board in between. Usually I go back after cells are accepted and pull the slide and then its a finisher. So I guess this hive overwintered two queens. I haven't noticed any supersedure cells in this hive or anything to make me think she is a new queen. How common is two overwintered queens in the same hive? Also a side note this has been my best hive for two years running I wonder how much of that time it has had two queens? Could that be a genetic trait or just something that happens from time to time. Anyone with experience on the subject.

I beleive according to larry conner's books, he say 10 percent of the time a hive combs outta winter with two queens.

For the beekeeper that does not have an annual requeening system it's probably quite common. back in the days when my dad and I ran the bees (150-200 hives) we didn't have a requeening program and quite often found 2 queens with in a hive, maybe 2 hives a year, but there were probably more then than that that we didn't notice. I requeen annualy now and even then I've seen a few.