Unfortunately what you have is not a mint error. It is the result of creating a "two headed dime" (novelty), and the two pieces come apart. For a double headed coin, a company or individual will grind off the reverse, then drill (hollow) out another coin from the reverse. The first coin is then placed insided of the second, with the head facing out, creating a two headed coin effect. The opposite is done with a two tailed coin. In my half dollar roll searches, I was surprised to actually find the hollowed out piece of the novelty coin, as the grinded off piece had come out. Your piece is the one that was grinded off then placed inside the hollowed out piece, but as with the half I found, they came apart. In my roll searches I have found a complete "two headed" half, a complete "two headed" nickel and the hollowed out piece of the two headed half.