You are one lucky dude. That coin is known as a "Mule". The reverse isn't supposed to exist for Constantine I, only for two of his sons (Constantine II and Crispus). In fact, that reverse does not appear to be listed anywhere in RIC. It should be DOMINORVM NOSTRORVM CAESS. Yours only states DOMINORVM NOSTRORVM. Very odd...
Your coin was struck in Ticinium. The obverse (heads side) is emperor Constantine The Great (Constantine I). The reverse (Tails side) is of either Crispus or Constantine II.
Therefore, it is an unlisted coin in the R.I.C. volumes and pretty rare. Mules are quite interesting. I would guess the mint workers were "changing over" product runs and forgot to change out one of the dies -> This is how I believe mules occurred. I do not own a mule.......